Of course if your changing Integration midstream you will have this issue. There is nothing really to do about it other than the admin editing each picture and reassigning to the proper user.
If you do things from a script level. You would need to rewrite the photopost entries to the proper usernumber and also rewrite all the files to use the new userid
Vb gave them.
We rely on userids to track everything so if you change forums etc and userid's change this will wreak havoc on Photopost