Mastodon supports post editing, you can edit any of your own posts any time by clicking ⋯ and then "Edit".
If you forget to add an image description, you can edit your post to add the description afterwards.
(Just mentioning this because still seeing people forgetting descriptions and adding them in replies, it seems they don't know about post editing.)
Lots more info and questions answered about post editing on Mastodon at:
