Image SEO: alt text, file size and names

Last updated 12 October 2026 · By the XKey team

What is alt text, and how do I write it?

Alt text is a short description stored with an image. Screen readers read it aloud to blind visitors, and Google uses it to understand what the picture shows. Describe the picture as you would to someone on the phone:

• Good: "New oak staircase fitted in a Victorian terrace in Lytham"
• Too vague: "stairs"
• Keyword stuffing: "joiner Lytham joinery staircases Lytham cheap joiner"

Keep it to a sentence. Purely decorative images, such as a background pattern, should have empty alt text (alt="") so screen readers skip them. Every website builder has an alt text box when you click on an image; the free SEO check lists any images that are missing it.

How big should images be?

Most photos straight off a phone are 3 to 5 MB – far too heavy. For a normal website, aim for:

• Width: about 1,600 to 2,000 pixels for full-width banners, and 800 to 1,200 for photos inside the text.
• File size: ideally under 200 KB for most photos, and well under 500 KB even for a big banner.
• Format: WebP or AVIF where your site allows it; JPEG for photos otherwise; PNG only for logos and graphics with flat colour.

Large images are the usual cause of a slow Largest Contentful Paint, one of Google's Core Web Vitals. Many builders shrink images for you, but they can't fix a 6,000-pixel upload completely, so resize before uploading.

Do image file names matter?

A little. A name like oak-staircase-lytham.jpg tells Google more than IMG_4821.jpg, and it's no extra effort if you rename photos before uploading. Use lower case words separated by hyphens.

What stops images making a page jump around?

Give each image its width and height in the code, so the browser keeps space for it before it loads. Without that, text jumps down as pictures appear, which annoys visitors and counts against you in Core Web Vitals. Most builders do this already; the SEO check flags images that don't reserve their space.

Also let images further down the page load lazily (only when someone scrolls near them), but never the main image at the top, which should load straight away.

Should text ever go inside an image?

Avoid it for anything important. Prices, phone numbers, opening hours and headings baked into a banner can't be read reliably by Google or AI assistants, don't resize well on phones, and are invisible to screen readers. Put the words in real text over or beside the picture instead.

Are my own photos better than stock photos?

For a small business, usually yes. Real photos of your work, team and premises build trust with customers, can appear in Google Images for local searches, and show the first-hand experience Google's guidelines value. Stock photos are fine for decoration, but they won't set you apart.

Frequently asked questions

Does alt text help SEO?

Yes. It's one of the main ways Google understands an image, it helps you appear in Google Images, and it makes your site usable for blind visitors.

How long should alt text be?

Usually one short sentence, around 125 characters or fewer. Describe what's in the picture, not a list of keywords.

Do I need alt text on my logo?

Yes, usually just your business name. If the logo links to your home page, that name is also the link's text.