What do they mean by posting in Cosplay post bar asking for the opposite picture?

The correct spelling of "reverse the picture" is "return the picture", which means to return the picture.

In cosplay, "seeking back the picture" means that coser collects photos featuring coser himself from the public.

Background: coser, as an exhibitor in the comic exhibition, usually has no photos of himself, but many people often take photos of him at that time. Coser wants to get feedback on his grades, and will post on the Internet to find his photos at that time.

Extension: "Seeking the map back" is not limited to coser's own photos, but also suitable for people who have never been to coser's photos. Anyone who wants any photos can seek to return them to enjoy them.