First, a terminology correction: “alt tag” isn’t quite right. It’s an alt attribute on an <img> element. It’s commonly called an “alt tag” across the SEO industry, so I’ll use both terms, but knowing the correct terminology helps when communicating with developers.
The alt attribute provides a text description of an image. It was originally created for accessibility: screen readers use it to describe images to visually impaired users. Google also uses it to understand what an image depicts, which affects image search rankings and provides context for the surrounding content.
<img src="kitchen-extension-leeds.jpg" alt="Completed kitchen extension with oak flooring and bi-fold doors opening to garden">
Alt text serves two audiences simultaneously. Screen readers rely on it to give visually impaired users a meaningful description of what they’d see if they could view the image. Google’s image recognition systems use it (alongside other signals) to understand image content, categorise it for image search, and connect it to the page’s topic. Good alt text for accessibility is almost always good alt text for SEO. The two goals rarely conflict.
Why Alt Text Matters
Accessibility (The Primary Purpose)
Approximately 2 million people in the UK live with sight loss. Screen readers, the software that reads web content aloud, rely on alt text to describe images. Without alt text, a screen reader either skips the image entirely or reads the file name, which is useless if the file is called IMG_4582.jpg.
Beyond being the right thing to do, accessibility is a legal consideration. The Equality Act 2010 and similar legislation in other jurisdictions require websites to be accessible. Missing alt text is one of the most commonly cited accessibility failures, and it’s one of the easiest to fix.
Image Search
Google Images is the second largest search engine in the world. Alt text is one of the primary signals Google uses to determine what an image shows and whether it’s relevant to an image search query.
If you sell products, provide services with visual outputs (construction, interior design, landscaping), or publish visual content (infographics, diagrams, photography), image search can drive meaningful traffic. Properly described images rank in Google Images; undescribed images don’t.
Page Content Context
Alt text also provides context for the page’s content. An image of a kitchen extension on a page about kitchen extensions reinforces the page’s topical relevance. Google uses every available signal to understand what a page is about, and alt text on relevant images contributes to that understanding.
This is not an invitation to stuff keywords into alt text. It’s recognition that descriptive, accurate alt text naturally includes topically relevant language.
How to Write Good Alt Text
Describe What the Image Shows
The fundamental rule: describe the image as if you were explaining it to someone who can’t see it.
Good: alt="Two-storey brick house extension with grey slate roof and full-height glazing"
Bad: alt="extension"
Bad: alt="house extension builders Leeds best extension company Yorkshire"
The first example describes what a person would see in the image. The second is too vague to be useful. The third is keyword stuffing that serves neither accessibility nor SEO.
Be Specific but Concise
Alt text should typically be 5-15 words. Long enough to be descriptive, short enough to not become a paragraph. Screen readers read alt text in a continuous flow; excessively long descriptions are tedious to listen to.
Good: alt="Open-plan office fit-out with exposed brick walls and pendant lighting"
Too short: alt="office"
Too long: alt="This image shows a recently completed commercial office fit-out project in Manchester city centre featuring exposed brick walls, industrial-style pendant lighting, polished concrete floors, standing desks, and collaborative workspace areas with modern furniture"
Include Relevant Keywords Naturally
If the image naturally relates to a keyword you’re targeting, include that keyword in the alt text. The operative word is “naturally.” If you’re describing the image accurately and it happens to include your keyword, that’s good. If you’re forcing a keyword into a description where it doesn’t belong, that’s keyword stuffing.
Natural keyword inclusion: On a page targeting “loft conversions,” an image of a completed loft conversion has alt text: alt="Completed loft conversion with velux windows and en-suite bathroom". The keyword appears because it accurately describes what’s in the image.
Forced keyword stuffing: On the same page, a stock photo of a person smiling has alt text: alt="loft conversion specialists affordable loft conversions UK". The image has nothing to do with loft conversions; the alt text exists only for keyword placement.
Don’t Start With “Image of” or “Photo of”
Screen readers already announce that the element is an image. Alt text that begins with “Image of…” or “Photo of…” is redundant. Just describe what’s shown.
Good: alt="Team of builders installing roof trusses on a residential new build"
Redundant: alt="Photo of team of builders installing roof trusses on a residential new build"
The exception: if the type of image is relevant to the description, include it. alt="Architectural floor plan showing ground floor layout with dimensions" is fine because “floor plan” describes the type of visual content, which is important context.
When Not to Use Alt Text
Not every image needs descriptive alt text. Decorative images that don’t convey information should use an empty alt attribute:
<img src="decorative-divider.png" alt="">
An empty alt="" (not a missing alt attribute, but a deliberately empty one) tells screen readers to skip the image entirely. This is correct for:
- Background textures and decorative elements
- Spacer images (these shouldn’t exist in modern web design, but they persist)
- Purely decorative icons next to text that already conveys the same information
- Stock photos used for visual interest rather than informational content
A missing alt attribute (no alt attribute at all) is different from an empty one. Missing alt attributes are an accessibility failure because the screen reader doesn’t know to skip the image and may attempt to read the file name. Always include the alt attribute, even if the value is empty.
Image SEO Beyond Alt Text
Alt text is the most important image SEO element, but it’s not the only one.
File Names
Name image files descriptively before uploading them. kitchen-extension-oak-floor.jpg is better than IMG_4582.jpg for both SEO and file management. Google uses file names as a secondary signal for image content.
Image Format and Compression
WebP is the preferred format for web images: smaller file sizes than JPEG or PNG with comparable quality. Most modern browsers support WebP. Use JPEG as a fallback for older browsers if needed.
Compress images before uploading. A 5MB image that displays at 800px wide on your page is wasting bandwidth. Resize images to their maximum display dimensions and compress them to the smallest file size that maintains acceptable quality. Tools like TinyPNG, ShortPixel, or Squoosh handle this.
Large images slow your page down, which affects Core Web Vitals scores and user experience. Image optimisation is often the single quickest page speed improvement available.
Lazy Loading
Lazy loading defers the loading of off-screen images until the user scrolls to them. This improves initial page load time because the browser only loads images that are actually visible.
<img src="project-photo.webp" alt="Completed warehouse conversion" loading="lazy">
Modern browsers support native lazy loading via the loading="lazy" attribute. Don’t lazy-load images that appear above the fold (the first screen the user sees); these should load immediately.
Image Sitemaps
If images are a significant part of your content (e-commerce product images, portfolio photography, project showcases), include them in your XML sitemap or create a dedicated image sitemap. This helps Google discover images that might not be found through normal crawling, particularly images loaded via JavaScript or embedded in galleries.
Structured Data for Images
For product images, recipe images, and other specific content types, structured data can connect images to their schema context. Product schema includes an image property that tells Google which image represents the product. This can influence which image appears in search results.
Common Alt Text Mistakes
Missing alt text entirely. The most common issue. Every informational image needs an alt attribute with a description.
Keyword stuffing. Writing alt text for Google instead of for users. Google’s guidelines explicitly warn against this, and it provides a poor experience for screen reader users.
Generic alt text. alt="image1", alt="photo", alt="banner". These are no better than missing alt text.
Identical alt text on multiple images. Every image should have unique alt text that describes that specific image. Copy-pasting the same description across 20 product images helps nobody.
Alt text on decorative images. Describing decorative elements clutters the experience for screen reader users. Use alt="" for purely decorative images.
Using title attributes instead of alt. The title attribute creates a tooltip on hover. It’s not a substitute for alt text and isn’t read by all screen readers. Always use the alt attribute.
Auditing Your Alt Text
To check your site’s alt text implementation:
- Crawl with Screaming Frog. The “Images” tab shows every image on your site with its alt text, highlighting missing alt attributes and images with overly long alt text.
- Check manually on key pages. Right-click an image, select “Inspect,” and check the
altattribute in the HTML. Or view the page source (Ctrl+U) and search for<img. - Use a screen reader. Install NVDA (free) or use VoiceOver (built into Mac) and navigate your site as a visually impaired user would. This is the most revealing test of whether your alt text actually works.
- Review Search Console’s image performance. Under “Search Results” in Search Console, filter by “Image” search type to see which images drive traffic and which keywords they rank for.
Alt text is one of those SEO elements that takes minimal effort to get right, provides genuine value for accessibility, and compounds over time as your images gain visibility in Google Image Search. If your site has hundreds of images with missing or poor alt text, fixing them is a quick win that a technical audit would typically flag as a priority.