Writing Accessible Images and Alt Text Correctly

Success criterion 1.1.1 Non-text Content requires that all non-text content has a text alternative that serves the equivalent purpose (W3C/WAI). The decisive word is purpose: an alt text does not describe the image as such, but the function the image has in its particular context. The same photo can be pure decoration on one page and carry central information on another. There is therefore no universally correct description, only one that is appropriate in context. The W3C Images Tutorials by the Web Accessibility Initiative distinguish several image types for this, each treated differently.

The practical relevance is high, because images are among the fastest-growing content types. In the WebAIM Million Report (2024), the average home page had 55.6 (WebAIM Million, 2024) images - an increase of about 28 percent over the previous year. Missing alternative text has for years been among the six most common WCAG failures, which together account for 96.4% (WebAIM Million, 2024) of all automatically detected errors. Describing images cleanly from the start avoids one of the largest and most easily measurable blocks of findings in a WCAG audit.

Before you write an alt text, you should determine the image type. The WAI essentially distinguishes between informative images, decorative images, functional images (such as logos in links or icon buttons), images of text, complex images and groups of images (W3C/WAI). Each type follows its own logic: a functional image describes the action ("start search"), not the appearance of the magnifier; an image of text should be replaced by real text in the first place. This classification determines whether you need a descriptive alt text, an empty alt text or a long description.

This system translates well into editorial processes. In our reference projects, it has proven useful to enforce a mandatory decision already when an image is uploaded in the content system: informative with alt text, or decorative with empty alt text. This prevents images without any alt attribute at all, which screen readers often read out as the file name. In addition, images are among the points that appear as a known gap in an honest accessibility statement with feedback mechanism as long as they are not yet fully described.

Good alt text is short, specific and related to the purpose. Bad alt texts, by contrast, are usually problematic in one of a few ways: they are empty where information is missing; they contain file names such as IMG_2381.jpg; they redundantly repeat the surrounding text; or they begin with filler such as "image of". The WebAIM Million Report (2024) notes that about one third of the images on popular home pages had missing, questionable or repeated alternative texts (WebAIM Million, 2024). A technically present alt attribute is therefore not yet good alt text.

There is no rigid length requirement in WCAG, but a proven rule of thumb: anything that cannot be described in roughly one sentence is usually a complex image and needs a long description. Pure length limits reported by individual tools are not a WCAG rule but a heuristic. More important than the character count is that the alt text hits the purpose and omits no information needed for understanding. When in doubt, the read-aloud test helps: would the text, read out, convey the same meaning as the sight of the image does for sighted users?

For purely decorative images - background patterns, dividing ornaments, mood photos with no message of their own - an empty alternative text is the correct solution (W3C/WAI). The attribute must be present but empty: alt="". If the alt attribute is missing entirely, some screen readers read out the file name or the image path instead, which disrupts operation. An empty alt attribute, by contrast, explicitly signals that the image should be skipped. For inline SVGs or images embedded via CSS, role="presentation" or embedding as a CSS background applies accordingly.

The distinction between decorative and informative is not always obvious. A mood image at the top of a blog article is usually decorative; the same image in a gallery that is precisely about that motif is informative. A helpful question is whether the page would lose information without the image. If the answer is no, an empty alt text is appropriate. We review such classifications as part of our services together with the editorial team, because they require editorial judgment and are not a purely technical criterion.

Charts, maps and infographics often carry more information than a short alt text can hold. The WAI recommends a two-step approach here: a short alt text that names what it is and summarizes the key message, plus a complete long description of the contained data elsewhere (W3C/WAI). The alt text of a bar chart could read: bar chart revenue 2025, rising from Q1 to Q4, data in the following table. The actual values then belong in the long description - ideally as accessible text or as a real data table directly in the page content.

There are several established routes for the technical link. A data table or an explanatory paragraph directly below the graphic is the most robust variant, because it is visible to all users without additional technology. Alternatively, the aria-describedby attribute links the graphic to a description text, or a figure with figcaption groups image and caption. Complex graphics are also a frequent point in the list of known barriers, which is why they are best considered together with other media - for example in the article on accessible media with captions and transcripts, where the same principle of a text alternative applies to audio and video.

Automated testing tools reliably detect whether an alt attribute is missing - but they cannot judge whether the text present is meaningful (W3C/WAI). A solid review therefore combines the automated scan with a manual inspection: reading the page with a screen reader immediately exposes file names, empty descriptions on informative images and redundant texts. A simple first step without special tooling is to disable images in the browser as a test and check whether the page remains understandable.

For alt texts to stay correct over time, they have to become part of editorial routine. From supporting 50+ (project experience) projects, we know that cleaning up afterwards is more effort than a clear rule at upload time. A short editorial guideline, a mandatory field or decorative marker in the content system and a regular spot check all help. We also anchor this interlocking of technology and editing through hands-on training for editorial and development teams, so that not only the current stock but every new piece of content stays accessible.

The business case also argues for describing images cleanly. In Germany around 1.2M (DBSV) blind and visually impaired people live, and about 4% (Federal Statistical Office, 2024) of people with severe disabilities have a vision impairment or blindness as their most severe condition. On top of this, meaningful alt texts and accompanying data tables improve discoverability in image search. Accessible images are therefore not a pure compliance item but expand the reachable audience - an argument we regularly pick up in our BFSG requirements.