In general, Web Content Accessibility Guidelines (WCAG) 2.0 guidelines also apply to social media when applicable.
How to choose images for social media
Most images posted on social media require accurate alternative text descriptions. The lack of accurate alternative text and Images of text are the primary barrier to making social media content accessible.
See the following examples:
Good example (no text in image)
The entire starting lineup is listed in text. The image is a photograph of the pitcher. The image also has alt text of "Duke pitcher Chillari holding a baseball".
Bad example (text embedded in image)
All of the statistics about the player is embedded in the image. The real text above the image does not replicate the text embedded in the image. None of the information in the graphic is available to screen reader users.