Images for teaching and learning must have alternative text provided, which visually describes essential information so that people with disabilities using assistive or adaptive technology can access the information. Understanding what makes good alt text is subtle and important. It should be brief: less than 250 characters. It should convey the purpose of the image, not describe the image.

• Avoid “image of”, “photo of”, etc, unless the medium is particularly important.
• Avoid using the title attribute in lieu of alt text. Keyboard-only users or mobile users may never see the title.
• Image links should describe the purpose of the link, and must never describe the image.
• Decorative images should have blank or empty alt text.
• Complex images, like charts or graphs, should have long description located near the image. The image’s alt text should describe where the close by image is.
• Posters, flyers, and the like must have the same information presented in nearby text.
• Provide color contrast and other design elements to help color blind users.
• Avoid flashing images. 

-https://usability.yale.edu/web-accessibility/articles/images