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 and generally less than 50 characters. In School Loop's CMS, where images can be inserted, you'll find an "Alternate Text" field in the toolbar. The following examples illustrate the most common use cases.
Drawings or Photographs
The alternate text for this image could simply be "Albert Einstein".
Images with Text
Use the text in the image as the alternate text.
alt="Go vote today"
Images Used as Links
If the image stands alone as the link, the alternate text should be the content of the image.
We suggest WebAIM's article on alternative text.