@pookie When you add a media, there is a button to add an Alternate textual description of the image.

(On my instance it is at the bottom left of the image, and it even lights-up with a little danger sign if there is not description).

This description can then be seen when hovering your image or clicking the Alt Button on it, and help for instance blind people understand what the image is. (Or people with poor network connection, etc).

When your image contains text, it can also help people translate it into another language too.

Note that you should be able to add and edit this text after the fact in principle.