Images and graphics make content more pleasant and easier to understand for many people, and in particular for those with cognitive and learning disabilities. They serve as cues that are used by people with visual impairments, including people with low vision, to orient themselves in the content.

However, images are used extensively on websites and can create major barriers when they are not accessible. Accessible images are beneficial in many situations, such as: 

• People using screen readers: The text alternative can be read aloud or rendered as Braille
• People using speech input software: Users can put the focus onto a button or linked image with a single voice command
• People browsing speech-enabled websites: The text alternative can be read aloud
• Mobile web users: Images can be turned off, especially for data-roaming
• Search engine optimization: Images become indexable by search engines
  
  

The alt attribute should typically:

• Be accurate and equivalent in presenting the same content and function of the image.
• Be succinct. This means the correct content (if there is content) and function (if there is a function) of the image should be presented as succinctly as is appropriate. Typically no more than a few words are necessary, though rarely a short sentence or two may be appropriate.
• NOT be redundant or provide the same information as text within the context of the image.
• NOT use the phrases "image of ..." or "graphic of ..." to describe the image. It usually apparent to the user that it is an image. And if the image is conveying content, it is typically not necessary that the user know that it is an image that is conveying the content, as opposed to text. If the fact that an image is a photograph or illustration, etc. is important content, it may be useful to include this in alternative text.
  
  

More information about accessible images.