Thanks for the suggestion! We agree that the iMacs have awesome displays and we wish we could take better advantage of them. For now, all our desktop backgrounds are generated from a single source, so we have to use a resolution that works on all library displays. The other complicating factor is that the resolution on the iMacs can be changed by anyone using them, so it's hard to determine the best resolution for the desktop images.
We're definitely aware of the issue though and won't give up on finding a workable solution!
Answered by: Amanda Etches-Johnson (User Experience Librarian)