I didn't know you could do that!

From what I can see, this means you can add alt text to images you embed using markdown. However, directly uploaded images in posts don't seem to have a way to directly add alt text that I can find. You can add a transcription in the text body, but it's not quite as accessible as pure alt text because screen readers won't read it automatically when the image gets focused.

Possibly you could embed an image in the text body and upload that like a text post, and that way use the markdown to add the alt text, but that would require the image is already hosted somewhere in the internet.