Repair permissions with Disk Utility (in Utilies).
That didn't work.
However, I think I did find the solution, which I am posting here in case someone has a problem similar to mine and Googles this up.
Background: After installing 10.5.7 on my wife's G5 iMac, the system crashed and would not reboot. I don't know what caused the problem, but I couldn't solve it so finally reinstalled Leopard from DVD, installed the 10.5.7 combo updater, and migrated from Time Machine. Everything seemed to be working except that System Preferences for Desktop and Screen Saver would hang with the beachball and have to be force quit. The iPhoto Library did NOT appear as an option for the desktop wallpaper or the screen saver.
NB: Still running iLife '06.
Attempts to solve: I tried many things that I found recommended on many forums. I erased plists, I reinstalled the combo updater, I reinstalled iPhoto, I copied a new iPhoto Library from another Mac, etc. Nothing worked, though occasionally the preferences worked partly so that I was able to get a picture up as wallpaper and select the entire iPhoto Library folder as screensaver -- unsatisfactory because it displays unmodified originals.
Solution: Following a hint in an Apple forum, after causing System Preferences to hang yet again I looked at the console. This is what it said:
6/18/09 4:09:08 PM System Preferences[4574] Call to deprecated method ILApertureMediaManager::init. Please update your code to use ILApertureMediaManager::sharedManager:
6/18/09 4:09:08 PM System Preferences[4574] Call to deprecated method ILiMovieMediaManager::init. Please update your code to use ILiMovieMediaManager::sharedManager:
6/18/09 4:09:08 PM System Preferences[4574] Call to deprecated method ILiPhotoMediaManager::init. Please update your code to use ILiPhotoMediaManager::sharedManager:
6/18/09 4:09:08 PM System Preferences[4574] Call to deprecated method ILiPhotoMediaManager::init. Please update your code to use ILiPhotoMediaManager::sharedManager:
6/18/09 4:09:08 PM System Preferences[4574] Call to deprecated method ILiTunesMediaManager::init. Please update your code to use ILiTunesMediaManager::sharedManager:
6/18/09 4:09:41 PM com.apple.launchd[96] ([0x0-0xc69c69].dotmacsyncclient[4583]) Exited with exit code: 156
Not knowing how to carry out the indicated updates, and figuring that since I'm still running iLife '06 a recent iLife update may be the source of the problem, I returned to Time Machine and restored the entire /System/Library/PrivateFrameworks/iLifeMediaBrowser.framework folder from last January, five months ago.
That appears to have solved the problem. Note again: iLife '06, G5, 10.5.7.
