I am experiencing the exact same problem. However, in my case, moving the iPhoto library to a disk image (as opposed to where it was, on my hard drive in the /Users/Shared area), was done in order to allow multiple users to access the library (not at the same time of course). So Bill's solution, of moving the library to another location, won't really solve my original problem.
Originally, I just made the library permissions r/w for everyone, and that seemed to work for a while. But then iPhoto started acting strangely and I suspected permission issues so I searched for a way to do it "properly" and found this article. However the Disk Image (where you can "ignore ownership") doesn't solve my issue because the other user still cannot access the library.
The only alternative I can see is to move the library back to the /Users/Shared folder and set the permissions as r/w for everyone... unless someone else has clarifications or suggestions?
iPhoto version 9.4.1 on Mac OS 10.7.5.
Thank you in advance,
Rami