But you can't add alt text to a picture you've already shared. You have to do it at the time of the post

When you upload the picture, before you post it, there is a little box in the top right corner of the image that says ALT. Click it and it'll prompt you to enter alt text.

It’s great that you’re sharing your favorite images! On Bluesky there’s a big push to provide alt-text and for accessibility, so you might end up blocked by some people if you don’t. Looking forward to seeing what you share