The ! or ? turns up when iPhoto loses the connection between the thumbnail in the iPhoto Window and the file it represents.
The most common cause of this is User Activity in the iPhoto Library Folder in the Finder.
Have you altered, moved or renamed anything in the iPhoto Library Folder?
If you haven't, then try rebuilding the database. Hold down the apple and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose the top three options.
Ig that fails, it might also be a file permissions issue: Download BatchMod from
http://macchampion.com/arbysoft/
And apply it to the iPhoto Library Folder using the settings found here:
http://homepage.mac.com/toad.hall/.Pictures/Forum/BatChmod.png
(Credit to Old Toad for this one).
Regards
TD