Images are great to help convey meaning in a quick way. However, when a user cannot view an image clearly that image can become a liability to a website's overall clarity and ease of communication. Fortunately, including well-crafted alt tags can fix this, but there is a bit of an art to writing clear, concise alt tags.
Identify the image in fewer than 140 characters as alt text. For example, "The Ecological Model. Four groups are stacked as progressively smaller ovals. See longer description in expandable/body of text." In the body of the page or expandable, provide a longer description such as: "The Ecological Model. Four groups are stacked as progressively smaller ovals. Each group covers some area of the previous group. The groups, from largest to smallest, are: Societal, Community, Relationship, and Individual." Include additional description as needed.
Include in alt text: "Bar graph. Title: Status of State Anti-Bullying/Harassment Laws 2003-2014. See expandable/body of text for long description." In the longer description, you may describe it in more detail. For example, "The x-axis is year 2003 to 2014 in increments of one year. The y-axis is number of states from 0 to 50 in increments of 5. ..."