Alt Attributes in SEO: A Complete Guide

Summary: Alt text (the alt attribute on an image) is a short written description of what an image shows. Screen readers speak it to people who cannot see the image, browsers show it if the image fails to load, and search engines and AI systems read it to understand the page. Good alt text describes the image plainly in a sentence or less, includes a keyword only where it fits naturally, and is left empty for purely decorative images.

What are alt attributes?

Alt attributes, commonly called alt text or alt tags, are a piece of HTML added to an image element on a web page. The attribute holds a text description of the image. In code it looks like this:

<img src="golden-retriever-puppy.jpg" alt="Golden retriever puppy playing in a garden">

Though often called the “alt tag”, that is technically wrong: alt is an attribute of the img tag, not a tag in its own right. The name has stuck and everyone knows what it means, so this guide uses alt text and alt attribute interchangeably.

Alt text is one of the oldest accessibility features on the web and one of the simplest pieces of on-page SEO. It is also one of the most neglected: in the technical audits we run before starting work on a site, missing or meaningless alt text is close to universal.

Why alt text matters

Alt text does three jobs at once, and the same description serves all three.

Accessibility

People who are blind or have low vision use screen readers that read the page aloud. When the reader reaches an image, it speaks the alt text. Without it, the user hears the file name, or nothing. Under the Equality Act 2010, UK businesses have a duty to make reasonable adjustments for disabled customers, and public sector websites are required to meet accessibility regulations. Alt text is the most basic step towards both.

It also serves everyone when an image fails to load, on a slow connection or a broken link: the browser shows the alt text in its place, so the page still makes sense.

A signal for search engines

Search engines cannot see images. They work out what an image shows from the file name, the alt text, the caption and the surrounding copy. Alt text is the most direct of those signals. A page about boiler repair whose images are described as “boiler engineer replacing a heat exchanger in a Preston home” gives Google more to work with than one whose images are all “IMG_4521”. It is a modest ranking signal for the page, and a stronger one for the image itself.

Google Images and image search results

Image search is a real traffic source for product, design, food, property and trade businesses. Images with accurate alt text, descriptive file names and relevant surrounding text are the ones that appear in Google Images and in the image packs that show at the top of ordinary results. Images without them rarely do.

Alt text and AI search

AI systems that answer questions and describe pages read alt text as part of the page. Accurate alt text on product and service images is one more way a business becomes legible to them, and one more reason to describe what the image actually shows rather than repeating a keyword.

How to write alt text

Describe the image as you would to someone on the phone who cannot see it. That is the whole skill. The rules below are refinements of it.

• Be specific. “Woman” is poor; “Dental hygienist showing a patient how to floss” is good.
• Keep it short. A sentence or less; most screen readers cut off around 125 characters.
• Do not start with “image of” or “picture of”. Screen readers already announce that it is an image.
• Include a keyword only where it describes the image. If the page is about kitchen fitting and the image shows a fitted kitchen, say so. If it shows the van, say that.
• Say what matters in context. The same photo of a shopfront might be “Studio 36 Digital office on Salmesbury Avenue, Blackpool” on the contact page and “Shopfront with new signage after rebrand” in a case study.
• Transcribe text in images. If the image is a chart, a quote or a sign, the alt text should carry the words or the key figure.
• Give every meaningful image alt text: product photos, team photos, diagrams, screenshots, logos, icons that carry meaning (a phone icon that means “call us”).

What makes a good image description

Take a photo of a dog trainer working with a border collie in a park, uploaded as DSC_0831.jpg.

The best version tells a screen reader user what is happening, tells Google what the image and the page are about, and reads like something a person would say.

Common alt text mistakes

• Leaving it blank on images that carry meaning.
• Using the file name as the description.
• Vague descriptions: “photo”, “image”, “banner”, “logo” with no name.
• Keyword stuffing: a list of search terms instead of a description. It reads badly, helps nobody, and Google treats it as spam.
• Repeating the caption word for word. The caption is visible; the alt text should add what the caption does not say.
• Describing decorative images at length (see below).
• Forgetting the logo. Your logo’s alt text should be your business name.

Decorative images: when to use an empty alt attribute

Some images are purely visual: a background texture, a divider, a stock photo that adds mood but no information. These should have an empty alt attribute (alt=””), which tells screen readers to skip the image entirely. Leaving the attribute out altogether is different and worse: without it, some screen readers read the file name instead. In WordPress, leaving the alt text field blank produces an empty attribute, which is the right result for decorative images.

Example: writing effective alt text

A dog-training business uploads a photo named IMG_2290.jpg to its puppy classes page. The image shows the trainer kneeling beside a golden retriever puppy on a lead, outdoors, with other owners and dogs behind.

1. Rename the file before upload: puppy-training-class-golden-retriever.jpg.
2. Write the alt text for the page it sits on: “Trainer working with a golden retriever puppy at an outdoor puppy class”.
3. Check it against the rules: specific, short, no “image of”, keyword present because the image genuinely shows a puppy class, reads naturally.
4. If the same photo is later used on the About page, rewrite the alt text for that context: “Our head trainer running a puppy class in Preston”.

How to add alt text

Most content management systems make this easy, and none of them do it for you.

WordPress

In the Media Library, open any image and fill in the Alternative Text field; it is saved with the image and used wherever the image appears. In the block editor, select an Image block and the Alt text field is in the right-hand sidebar under Settings; a value entered there applies to that placement only, which is how you give the same image different alt text on different pages. Rank Math and Yoast both flag images without alt text in their page analysis. Breakdance and other page builders have an alt text field in each image element’s settings.

Shopify, Squarespace, Wix

Each has an alt text field on image settings: Shopify in the product media editor (“Edit alt text”), Squarespace in the image block’s settings, Wix in the image settings panel under “What’s in the image?”. Product images deserve the most care; they are the ones that appear in image search and shopping results.

Alt text in Word, PowerPoint and PDFs

The same principle applies outside the web. Word, PowerPoint and Google Docs have an Alt Text field on every image (right-click the image, or use the Picture Format tab), and it carries through when the document is exported to PDF, provided the export keeps tagging on. Describe what the image shows for someone who cannot see it, and mark decorative images as decorative so screen readers skip them.

Tools and resources

Final thoughts

Alt attributes are far more than an SEO checkbox. They make your site usable for people who cannot see it, they keep pages readable when images fail, and they tell search engines and AI systems what your images and pages are about. Write a plain description of what each image shows, use a keyword where it belongs, leave decorative images empty, and name the logo. It takes a minute per image and it is one of the few on-page tasks with no downside. Image optimisation is part of every campaign in our SEO packages.