On-Page SEO

Image SEO: How to Write Alt Text and Optimise Images for Speed

By SEOKIFY Team· · · 2 min read

Images help pages rank and convert, but they are also the biggest cause of slow pages. Image SEO is about describing them well and shipping them lean.

Write useful alt text

Alt text describes an image for people who cannot see it and gives search engines context. Describe what the image shows and why it matters on the page.

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Avoid keyword stuffing and phrases like "image of". Find missing alt text with the Image Alt Text Checker.

Decorative images

If an image adds nothing, such as a divider, use alt="" so assistive technology ignores it.

Use descriptive file names

Name files like keyword-density-checker.png, not IMG_0042.png. Build clean names with the URL Slug Generator.

Compress and choose the right format

  • Resize images to the largest size they are displayed at.
  • Use WebP or AVIF where supported, JPEG for photos, and PNG only when you need transparency.
  • Compress before uploading.

Lazy-load and prevent layout shift

  • Add loading="lazy" to below-the-fold images, but not to your main hero image.
  • Set width and height so the browser reserves space and the layout does not jump.

Measure the effect with the Page Speed Checker, and read about the related metrics in our Core Web Vitals guide.

Frequently asked questions

How long should alt text be?

Keep it short and descriptive, usually a single sentence. There is no official limit, but concise text is easier for screen readers.

Should every image have alt text?

Meaningful images should. Purely decorative images should use an empty alt attribute so screen readers skip them.

Is WebP better than JPEG?

WebP usually produces smaller files at similar quality, and AVIF can be smaller still. Use modern formats with a fallback where needed.

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