Screen readers make use of the alternative text for images. As such, the alternative text should be descriptive of the content and function of the image. In School Loop's CMS, where images can be inserted, you'll find an "Alternate Text" field. The following examples illustrate the most common use cases.
Drawings or Photographs
Describe the image in 50 characters or less. The alternate text for this image could simply be "Albert Einstein".
alt="Albert Einstein"
Images with Text
Use the text in the image as the alternate text.
alt="Go vote today"
Images Used As Links
The alternate text should describe the destination for the link.
alt="Visit the Department of Education Website"
More Examples
We suggest WebAIM's article on alternative text.