Image descriptions are known as text equivalents (or alt-text), and are phrases, sentences, or combination of these that are associated with a particular image. 

Text should describe the image's visual appearance; and it can include instructions on how to take an action, such as navigating through online Help by clicking an arrow image. Any non-text element in your content should have a text equivalent. 

The information being conveyed will be different depending on the situation.

Take for instance the photo included in this content, a red beta fish with it's fins fanned out on a black background. This is the text description used for this content. Yet maybe for another course they are talking about color contrast between the black and red. Maybe another course is talking about the kind of beta, and yet another is talking about the pattern the fins are making. Each situation would call for a different text equivalent to go with the photo. 

As the author of a document, it is your responsibility to provide the text equivalents as you know what is trying to be conveyed.