Images can be useful to illustrate a point, but they should not be used as a replacement for a clear text description.
When publishing charts as image files, the underlying data must be available as a download for users who cannot see the image.
Format and size
Images must be no more than 600 pixels wide but can be longer than 600 pixels. Always consider usability when determining the length of an image; it needs to be fully visible on-screen without too much scrolling.
Wherever possible, provide images in SVG format so that users can magnify the content.
Text in images
Do not use text in images. If it cannot be avoided, the same text must be explained as a long description nearby on the page or in the alt text. This is so that it can be changed into other forms such as braille, speech or symbols.
Logos should not be included. If they must be included to show a collaboration with another department or organisation, there must be accompanying text naming the organisation. Logo images do not need alt text as this would repeat the text.
Use of colour
Avoid using colour alone to convey meaning in an image, as users with visual disabilities can find it hard to distinguish between them. Using red and green together is a common example as many people are unable to tell these colours apart. If you have to use colour to help explain something – check it with colour blindness software.
Maps with multiple clickable areas should provide an explanation in the alt text or in the text nearby that gives an overall context to the meaning of the map.