Terence - Thanks again. I have created a copy of the library on an external, and I have tried to rebuild the original library as you suggest - rebuild went fine, but - no luck - same result for my son's downloads - can see the thumbnail but opening resuts in black screen / exclaimation point.
Regrettibly I think this is a software glitch in iPhoto and with the mac and thus my frustration evident in my other post. The mac has permission restrictions built into multiple user profile setups, and the software will nevertheless seem to download, but doesn't really do so from a user profile which doesn't have permissions. (i.e. anyone other than the 'owner' of the library)
It seems Apple recommends to avoid this to use an external, which is not permission protected. See http://support.apple.com/kb/HT1198
Again, wish I'd been told this or found this 10 months ago when I went to set this up , not now after my son lost 400 images.
With 'genius bar' assistance, I even examined all the master jpegs inside the iPhoto package. The missing images simply don't seem to have downloaded, despite seeing the thumbnails.
I'm just sayin' .....doesn't seem right to me.