Quick answer
Image SEO starts with a useful page, a discoverable image URL, accurate alt text, and a stable layout. NOTABIS provides separate checks for missing or present alt attributes, image dimensions, legacy extensions, and related resource behaviour. Optimise formats and loading only after confirming the image’s purpose, quality, and accessibility.
What the issue means
An image’s filename, src, srcset, dimensions, format, alt text, caption, and surrounding copy each serve a different purpose:
<img src="trail-shoe.webp"
width="1200" height="800"
alt="Red trail running shoe on a rocky path">
Informative images need a concise text alternative. Decorative images should use alt="". Width and height reserve space, while responsive sources choose an appropriate file for the viewport. A caption can add context but does not replace alt text.
Why it matters for SEO
Search systems use image context, page text, and accessible alternatives to understand images. Fast, stable delivery improves user experience, but a modern extension is not a ranking guarantee. Keyword-stuffed alt text, misleading filenames, or a visually sharp image that fails for keyboard and screen-reader users is not a useful optimisation.
How NOTABIS detects it
NOTABIS reports missing image alt text when img elements lack an alt attribute and passes image alt checks when none are missing. It flags missing dimensions when width or height is absent and records legacy image formats for .jpg, .jpeg, .png, and .gif URL extensions. These are markup and extension heuristics: they do not judge wording, CSS backgrounds, srcset, actual MIME bytes, visual quality, or Core Web Vitals.
How to check it yourself
Inspect the rendered DOM and accessibility tree, then request representative image URLs. Check Content-Type, transfer size, dimensions, compression, caching, and responsive selection in the Network panel. Ask what information a user loses if each image is unavailable. Test lazy-loaded and JavaScript-rendered states as well as the initial HTML.
How to fix it
Write concise alt text that describes the image’s purpose, use empty alt for decoration, and keep important explanations in visible text. Provide intrinsic width and height, responsive sources, and a format pipeline that preserves quality. Convert suitable photographic assets to WebP or AVIF with a fallback, but keep PNG or GIF when transparency, animation, or compatibility requires it. Lazy-load below-the-fold images without delaying the main image or hiding content from users.
Recheck visual quality, layout stability, keyboard and screen-reader output, and actual performance after each change.
When this is intentional or does not need fixing
Decorative backgrounds, logos whose name is already present as text, screenshots, animated images, and third-party embeds may need different handling. A legacy format can be correct for transparency or compatibility, and a small image may not justify conversion. Do not remove a useful image or replace meaningful alt text with a keyword list.
Related issues
Read image alt text checks, missing image alt text, missing image dimensions, and legacy image formats. Google’s image SEO guidance covers image context and text alternatives.
Run a NOTABIS audit
NOTABIS combines image markup evidence with page-level findings so you can prioritise accessibility fixes and performance work that benefits real users.