As far as I can tell, there is no way to move images from one member's gallery (a subcategory) to another member's gallery. The Mass Move option only allows you to pick a UserID (I can't even tell whether it is source or dest category, duh).
Is there any other way to do this? If a VB user changes their name, the gallery obviously doesn't move by itself, but we should have a way of cleaning this up (until the auto cleanup option is available of course!).