If you're referring to adding image descriptions to images you upload, – when you're making a new toot and have uploaded an image – the images you've uploaded should say Edit on them (before you've posted your toot, yet).
Click the Edit on the particular image and you'll be able to add an image description; this allow screen readers to get the description of your image you've provided, for those who use screen readers to browse the web.
@Swanye @rysiek oh right, how much should I describe the image? Or is it just the main idea of the image. Like I post one of a spider, would I write ‘spider crawling’ or of a drawing ‘drawing of squirrel.’ Something like that?
Tbh think I’m going to have to do some research on this. Haha thanks for replying! :ablobblewobble: