If you want to do this on mobile devices, it depends on your app.
in Mastalab, Tusky and others you would need to tap the picture and the line "describe for the visually impaired" will appear, and you just have to tap the screen there and enter your text.

#accessibility
#addyourdescriptions
#letsbemoreinclusive

In #Amaroq, which I use, the text area is displayed after uploading the picture, I don't even have to tap it:

hey, seeing folks, I'm mostly not boosting toots with pictures in them that have no description.. and sadly I see a lot where there isn't one.

it's easy, just hover your cursor over the picture or tap it and you will see a line with a text:
"describe for the visually impaired". click/tap on this line and enter a short description (or long if you prefer, because it doesn't count towards the text of the toot).

tadaaaa you're done!

#addyourdescriptions
#letsbemoreinclusive
#accessibility