Mastodon also has the ability to add image descriptions to your media posts, and we highly recommend you add them whenever possible.
When you upload an image, simply click the "Edit" button to be able to change its crop and add a description. It doesn't have to be too detailed, even a single sentence to get the idea across is helpful.
This goes a long way to helping make our community more accessible to folks who use screen readers. 
If you want to make an effort to include descriptions but keep forgetting, feel free to follow @PleaseCaption; it will send you a friendly DM whenever you forget to add a description, allowing you to Delete & Redraft before the post gets too many notes.
Adding a description helps a lot more people than just those who use screen readers, too! If you're able, please consider adapting it as a habit whenever you post.
Thank you! 
@Curator hi! i want to ask if putting an image description is enough in the alt text, or it should be part of the body of our toot?
@naomifrisson The alt text is just fine! 
@Curator perfect thnx š¼
If you'd like some ideas of how to write an image description/alt text, you can refer to this handy guide: https://www.americananthro.org/ImageDescriptions
There are many other guides out there, too, if you do a quick internet search.
If anyone who regularly appreciates or relies on image descriptions want to reply with their own tips/tricks/links/resources, please do! I am by no means an expert on such things!