...when you mark image 12345 as your 'show this image in your gallery' image, you are in fact adding a tag for this game with the name 'own:12345'.
Ah, so you don't need to actually click on the "own:xxxxx" tag in the Popular Tags section. So these own: tags show what some other people are using for the representative image for this game in their collection. Cool.
Yes... They are talking to you!
... I shoulda seen this one coming...