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THOUGHTS / BEGINNERS GUIDE 1 Jul 2025 8 MIN READ

What Are Image Alt Tags and Why Does Your Website Need Them?

Every image on your website can carry a short piece of hidden text called an alt tag. Google reads it to understand what the image shows. Screen readers read it aloud to visually impaired visitors. Most small business websites get this wrong — either leaving alt tags completely empty or stuffing them with keywords in a way that helps no one. Here is what alt tags actually are and how to write them properly.

An alt tag (short for alternative text) is a brief text description of an image, written into the website code. It serves two purposes: it tells Google what the image contains, helping with SEO, and it provides a text alternative for visitors who are blind or have images turned off. Good alt text describes the image plainly and specifically — "a white ceramic coffee mug on a wooden table" rather than "image" or "photo". Decorative images that add no information — such as dividers and background patterns — should have empty alt text so screen readers skip them.

There is a quiet assumption built into most small business websites: that the photographs speak for themselves. They do — to a human being who can see them. But Google cannot look at a photo the way a person can. When its crawlers visit your website, they read the code — including the alt text attached to each image — to understand what is on the page. An image of your team installing solar panels on a roof is, to Google, an unmarked file. With a good alt tag, that same image becomes evidence that your page is genuinely about solar panel installation, which helps you rank for related searches.

The accessibility angle matters just as much, and it is worth pausing on. An estimated two million people in the UK live with significant sight loss. Many of them use screen readers — software that reads web pages aloud. When a screen reader reaches an image with no alt text, it either says nothing or reads out the file name, which might be something like "IMG_4721.jpg". That is not information. It is noise. A well-written alt tag means those visitors receive the same understanding of your page as everyone else — which is both a legal consideration and, more plainly, the right thing to do.

Where Do Alt Tags Live?

In the underlying code of a web page, images are added using an HTML tag that looks something like this:

<img src="team-photo.jpg" alt="Three plumbers in uniform standing outside a white van">

The alt="..." part is the alt tag. You will never need to write this yourself. On platforms like Squarespace, Wix, or WordPress, there is a field labelled "Alt text" or "Alternative text" whenever you upload or select an image. On a hand-coded site, your developer handles it — but it is worth knowing how to find and update it, because the responsibility for keeping it accurate as your business evolves sits with you.

How to Write Good Alt Text

The instinct, when writing alt text for the first time, is either to say too little or too much. People write "photo" and leave it at that, or they list every keyword they would like to rank for, hoping Google will be impressed. Neither works. The actual goal is simpler: describe what is in the image as you would explain it to someone on the phone. Around ten to fifteen words is usually enough. Do not begin with "image of" or "photo of" — screen readers already announce that it is an image, so that phrase adds nothing.

Four principles are worth keeping in mind when writing alt text, and they are worth naming plainly:

  • Be specific. "A red Ford Transit van with 'Smith Plumbing' written on the side" is better than "van" or "company vehicle".
  • Include relevant context. For a product image, include the product name, colour, and key features: "Navy blue linen cushion cover with white geometric pattern, 50cm square".
  • Do not stuff keywords. Writing "plumber London emergency plumbing pipes" as alt text is spammy, unhelpful to disabled visitors, and can actually harm your SEO. Describe the image naturally — if the image is relevant to your business, keywords will appear naturally in the description.
  • Leave purely decorative images empty. If an image is purely decorative — a coloured background, a dividing line, a generic stock photo that adds no meaning — the correct approach is to give it an empty alt attribute: alt="". This tells screen readers to skip it, which prevents them from reading out meaningless descriptions.

Good alt text is not a caption and not a keyword list. It is the answer to one quiet question: what would a person miss if they could not see this?

Does Alt Text Directly Affect Google Rankings?

For your main search rankings, alt text is one small factor among many. Where it makes a more visible difference is in Google Image Search — and that matters more than people realise. If you run a business where customers search visually — a florist, a furniture maker, a photographer, a baker — every unlabelled image is a missed opportunity to be found. Good alt text on every image increases the chance that those images appear when someone searches Google Images.

Beyond rankings, alt text is increasingly important for AI-powered search tools that interpret the content and context of your whole page. These systems do not just read your paragraphs — they read the signals attached to every element, including images. The more clearly labelled your page is in its entirety, the better AI-driven search understands what your business does and when to surface it to a potential customer.

How to Check Your Website's Alt Text

If you are curious about the state of your own site, checking is easier than it sounds. Right-click on any image on your website and select "Inspect" in Chrome or Firefox. Look for the alt="..." attribute in the code that appears. If it is empty or missing entirely, the image has no alt text. If it contains the file name or a string of keywords rather than a plain description, it needs rewriting. No technical knowledge is required to read what you find there.

For a fuller picture across your entire site, free tools like Google's Lighthouse (built into Chrome's developer tools) and WebAIM's WAVE accessibility checker will flag images with missing or poor alt text in one pass. Both are worth running, and both are free.

A Common Mistake to Avoid

One of the most frequent alt text errors on small business websites is using the image file name as the alt text — something like "DSC_0042.jpg" or "homepage-banner-v3-final.png". This happens quietly, without anyone noticing: images are uploaded without filling in the alt text field, and the platform falls back to whatever the file was named on someone's camera or desktop. It provides no useful information to Google or to a screen reader user. If you suspect this has happened on your site, going back through your images and adding proper descriptions is straightforward work that pays for itself quickly in both accessibility and search.

Frequently asked

Is alt text the same as an image caption?
Not quite. A caption is visible text beneath an image that any reader can see — it adds context, tells a story, names a place. Alt text lives inside the code and is never visible on the page; it is read aloud by screen readers or displayed only when an image fails to load. They serve genuinely different purposes, and you can have both. A caption that says "Our team at the Bristol project, March 2026" and an alt tag that says "Four builders in hard hats standing on scaffolding outside a red-brick building" are doing separate jobs, and neither replaces the other.
How long should alt text be?
Under 125 characters — roughly one or two short sentences. Screen readers typically stop reading at that point, so anything longer simply disappears. For most photographs, ten to fifteen words is sufficient and more is rarely better. The exception is a complex image like a detailed infographic, where the image itself carries essential information. In that case, a longer description belongs on the page as visible text, with the alt text offering a brief summary: "Infographic showing five steps to planning a loft conversion."
Do I need alt text on every single image?
Every image needs an alt attribute in its code — but not every alt attribute needs words inside it. Images that carry meaning (photographs, product shots, diagrams, your team on a job) should have descriptive alt text. Images that are purely decorative — background textures, dividing lines, icons that are already described by text next to them — should have an empty alt attribute (alt="") so screen readers pass over them silently. The distinction matters: a screen reader that announces every decorative flourish on a page is exhausting to listen to, and makes the genuinely useful content harder to find.
Will adding alt text really make a difference to my Google ranking?
For main page rankings, alt text is a small factor among many — improving it will not transform your position overnight. Where the difference is more visible is in Google Image Search, which matters considerably for businesses where people search visually: interior designers, florists, bakers, photographers. Beyond search, though, there is the accessibility question, which is worth taking seriously on its own terms. The Equality Act 2010 applies to websites, and an inaccessible site carries genuine legal risk for UK businesses. Alt text is one of the simplest improvements available — a small effort with reach that extends well beyond rankings.

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