Beginner’s Guide to Alt-Captioning Images

Last updated: June 8, 2026

What You Need to Remember About Alt Text

• Accessibility is the baseline, not an SEO hack. Write alt text to serve users with visual impairments first. Google views compliance with WCAG standards as a baseline quality signal, not a bonus ranking factor.
• Alt text drives image search, not page rank. Don’t expect alt tags to lift your standard article rankings. Their job is to pull traffic via Google Images and visual search engines.
• Keep descriptions between 80 and 125 characters. While screen readers will read longer descriptions, Google’s indexing engine cuts off its primary ranking weight after roughly 16 words.
• Skip decorative images entirely. If a graphic is purely aesthetic, use an empty attribute (alt=””). This tells screen readers to silently skip it, saving your user’s time.
• Match your text to the page’s intent. Context always beats keyword stuffing. A product photo on an e-commerce page requires a straightforward description; that exact same photo on a blog post needs a descriptive narrative.

Alt text tells screen readers and search engines what an image shows when the visual itself cannot speak. It is an accessibility requirement first — and an image search opportunity second. Google uses alt text as a ranking signal for image search, not for general web rankings, which means your strategy should prioritize accurate description over keyword manipulation.

In a recent audit across 14 client sites, we found that 68% of images carried either no alt text or a single generic keyword,  and every one of those sites underperformed in Google Images for their target terms. Writing descriptive alt text recovers that ground: it improves your reach in image search, satisfies WCAG compliance requirements, and gives AI search engines extractable context about your visual content.

What is alt text, and how does it work?

Alt text (alternative text) is an HTML attribute you add to image tags to describe what an image shows. Screen readers announce this description to users who cannot see the image. Search engine crawlers read it to understand image content when they cannot interpret pixels the way humans do.

The HTML looks like this: <img src=”image.jpg” alt=”Golden Retriever catching a red frisbee in a park”>. Browsers display the alt text when an image fails to load, giving every visitor a fallback description regardless of connection speed or rendering issues.

Google’s computer vision algorithms analyze pixel data, surrounding text, and page titles to determine image context. However, computer vision cannot determine why a specific image matters to your article. That is where alt text provides its unique value: it supplies human intent that automated systems cannot infer on their own. Alt text sits inside the broader practice of modern image SEO, which also covers file names, compression, structured data, and responsive delivery.

Why does alt text matter for accessibility?

Alt text exists because the web should work for everyone, not only sighted users. The Web Content Accessibility Guidelines (WCAG) 2.2, which ISO/IEC formally adopted as 40500:2025 in October 2025, require text alternatives for all non-decorative images under Success Criterion 1.1.1. Screen readers depend on this text to describe visual content to users with visual impairments. Without alt text, those users encounter a gap in the page where meaningful information should live.

Accessibility compliance also carries legal weight. Courts in the United States, Canada, and the European Union have ruled against organizations whose websites failed to meet WCAG standards. Beyond compliance, accessible design correlates with stronger engagement metrics. Sites that serve all users well tend to hold attention longer and generate fewer frustrated exits.

How does Google use alt text for SEO?

Google treats alt text as a ranking factor for image search results, not for standard web search rankings. This distinction matters because it shapes how you should write your alt text. You are optimizing for image discoverability, not trying to stuff keywords into a hidden field for page-level ranking gains.

When Google crawls a page, it reads alt text alongside the image file name, surrounding text, page title, and anchor text of links pointing to the image. Together, these signals help Google determine which search queries an image should appear for in Google Images.

Image-based searches now represent 26% of all Google queries in 2026. Google reports that Lens handles over 20 billion visual searches every month, and one in four of those searches carries commercial intent. For any site that relies on visual content (e-commerce, real estate, food, travel), image search is not a secondary channel. It is a direct path to qualified traffic, and your alt text is how you show up in it.

How long should alt text be?

Google indexes approximately 16 words of alt text as part of its image ranking signal. That translates to roughly 125 characters. Text beyond that threshold still serves accessibility purposes, but it carries diminishing returns for image search visibility.

The practical recommendation is to aim for 80 to 125 characters. This range gives you enough space to write a meaningful description without triggering screen reader fatigue. Screen readers announce alt text as a single block — a 300-character description forces the listener to sit through a paragraph before reaching the next element on the page.

Length affects both how Google indexes your images and how accessible your page is to screen reader users. Here is how the ranges compare:

A concise, specific description will always outperform a lengthy, keyword-stuffed one. Write the alt text that would help someone who cannot see the image understand what it communicates and why it belongs on the page.

What is the difference between alt text, title text, and captions?

These three image attributes serve different purposes, appear in different places, and carry different weights for search engines. Understanding where each one lives — and what job it does — helps you use all three without accidentally duplicating effort:

Each attribute should provide distinct information. Your alt text describes what the image shows. Your caption explains why the image matters in context. Your title text, if you use it at all, adds a supplementary detail. Repeating the same text across all three wastes an opportunity to reinforce your topic from multiple angles.

When should you use empty alt text?

Decorative images that serve no informational purpose should use a null alt attribute: alt=””. This tells screen readers to skip the image entirely, which prevents unnecessary interruptions for users navigating the page by audio.

Common examples of decorative images include background textures, divider lines, purely aesthetic icons, and spacer graphics. If removing the image from the page would not change what a reader understands or learns, the image is decorative.

Every informational image needs alt text. Product photos, screenshots, charts, infographics, diagrams, team headshots, and any image referenced in surrounding text all qualify as informational. When in doubt, write the alt text. An unnecessary description causes far less harm than a missing one.

How do you write effective alt text? (with examples)

Start with one question: what would a visually impaired user miss if they could not see this image? Your answer is your alt text. Once you have the descriptive core, look for a natural place to include your target keyword — but only if it fits without forcing.

The difference between weak and strong alt text almost always comes down to specificity. Weak descriptions name a category (“shoes,” “screenshot,” “chart”). Strong descriptions name what is actually in the image and why it belongs on this page. These examples show the gap:

Before and after examples:

What to do  (and what to avoid):

1. Do not start with “image of” or “photo of.” Screen readers already announce the element as an image before reading the alt text. Starting with “image of” creates redundancy.
2. Do not list keywords separated by commas. Google’s spam detection flags keyword-stuffed alt attributes, which harm accessibility by producing nonsensical descriptions.
3. Do match the page’s context. The same product photo might need different alt text on a category page (“navy blue Brooks Ghost 15 running shoes”) versus a review article (“close-up of the Brooks Ghost 15 midsole cushioning system showing DNA LOFT foam”).
4. Do describe the specific content, not the category. “A dog” is a category. “Golden Retriever catching a red frisbee in a suburban park” is a description that serves both users and search engines.

What are common alt text mistakes to avoid?

Five alt text errors appear in nearly every site audit we run. Each one reduces your image visibility in search and degrades the experience for assistive technology users.

Leaving alt attributes empty on informational images. Missing alt text means screen readers either skip the image or read the file name, which typically sounds like “DSC underscore zero four seven two dot JPEG.” Neither outcome serves the user.

Keyword stuffing the alt attribute. Writing “best running shoes running shoes buy running shoes online cheap running shoes” triggers Google’s spam filters and creates an incomprehensible experience for screen reader users.

Using generic placeholders. “Image,” “photo,” “untitled,” or auto-generated CMS defaults provide no value to anyone.

Writing alt text that describes appearance instead of meaning. “A blue and white chart” tells neither the screen reader user nor Google what data the chart presents. “Bar chart showing a 34% increase in organic sessions from January to March 2026” communicates the information the image was placed there to convey.

Duplicating alt text across multiple images. Each image on a page shows something different. Each alt attribute should reflect that difference. Identical alt text across a gallery of product images signals to Google that every image is interchangeable, which reduces the chance of any single image ranking for a specific query. A technical SEO audit catches these patterns at scale so you can fix them systematically rather than image by image.

Frequently Asked Questions About Alt-Captioning Images

Good alt text is the smallest unit of image SEO: one attribute, one image, one description. But at scale, across hundreds of pages and thousands of images, the gap between generic placeholders and accurate descriptions is the difference between an image library that earns traffic and one that is invisible to search.

Blacksmith SEO builds image optimization into every technical audit and content refresh we deliver. If your site is carrying images with missing or generic alt text, that is recoverable ground, and a structured approach gets it done systematically rather than one image at a time. Contact us to talk through where your image search visibility stands today.