Alt text is used primarily for screen readers to describe images to visually impaired readers. It also appears in place of an image if the webpage fails to load for a variety of reasons. Alt text should not be used as a secondary place to have a caption. It should be succinct but descriptive. There is no hard rule on how long or short alt text should be, but it should be long enough to give enough specifics and context, without becoming too long for a screen reader to read through. 

A good goal to aim for is around 125 characters.

There are different use cases for alt text, so be mindful of its surroundings. Decorative images provide visual appeal to the page, but are not important. They still need alt text describing them. Linked images do not need “Link to” because the screen readers will already note if there is a link, so it should be descriptive all the same. If there is a caption or title adjacent, do not just repeat the title or adjacent text.

Image alt text needs to be specific and descriptive that adds context that relates to the topic of the page. Don’t start with “An Image of” or “A photo of” as it is going to be identified as such with the screen-reading tools.

For in depth examples and more comprehensive descriptions, please visit the webaim site.