As Yellow has surmised, it sounds like a problem with permissions,when I did the exact same thing after I messed up my photos, it just restored over it, no problems.
If you have another admin account you can see what the permissions on the library are and then maybe change them to what they should be.
For me, and I've done nothing with permissions on teh library, they are read and write for; everyone, staff, and myusername.
In fact, perhaps thats a good place to start, post what yours are.