Thank you. The alt text the OP provided doesn't help visually impaired people at all. This actually explains what's on the poster. Bravo!
@graciegato @GreenSkyOverMe Metatext is no longer supported by its creator, so you'll probably need to switch to a new app at some point.
In Metatext, though, what you can do it to edit the post, temporarily remove the image, and then immediately add it again. Once you do that you have the ability to add alt text.