Social Media Accessibility Toolkit Hackpad - a little outdated, but still a good source! https://digital.gov/resources/federal-social-media-accessibility-toolkit-hackpad/
A practice form for writing image descriptions from the American Anthropological Association. https://docs.google.com/forms/d/e/1FAIpQLScow49pNuUAF0RiYTFG7KT90UMbZifi-C9dP7TSsk5w0OV3Aw/viewform
Please visit our Accessibility Guidance page for more that can be applied to social media and / or longer form content.