I guess I just don't agree with "If they can use phpmyadmin then they can run a query." I think there are plenty of people who will use a gui interface but don't know and really don't want to learn the command line syntax necessary to accomplish what they want to get done.
I also think it's interesting that we are now eight replies into this thread and no one has ventured the SQL. So, here is what I came up with.
UPDATE cpg_pictures SET owner_id = 6, owner_name = 'test', aid = 28 WHERE aid = 29
It seems like the album id ("aid") also needs to be changed to move the image. I didn't find the user's albums to be accessible to the admin during upload or during a reassignment of the album to which an image belongs.
And, of course, substitute your own appropriate values for the field entries!
I don't know and use SQL but it seemed to work ok. (Comments welcome!)
In doing this I realized another issue with just changing the owner_id and owner_name. That is that the uploaded files aren't moved. They will NOT be in the user's "userpics" folder when you are done. (Keeping them together seems like a good idea.) The files could be moved manually and the SQL could be expanded to reflect this. But now it seems things are getting a bit more complicated. I am starting to like my original suggestion of just BEING the user for a little while!
In fact really all this "suggestion" was about was a way to BE a user for a little while in a pretty benign manner that the user probably will never be aware of. Use it, don' use it, whatever.