If permissions repair didn't fix it, try this:
ls -la /Library/Desktop\ Pictures/
Make sure there are some sort of whacked out permissions. If so:
sudo chown -R root:admin /Library/Desktop\ Pictures/ && sudo chmod -R 644 /Library/Desktop\ Pictures/
The above chmod to 644 will do the directory recirsively so, you will then want to go behind and do:
sudo chmod 774 /Library/Desktop\ Pictures/Art
sudo chmod 774 /Library/Desktop\ Pictures/Black\ &\ White
sudo chmod 774 /Library/Desktop\ Pictures/Nature
sudo chmod 774 /Library/Desktop\ Pictures/Patterns
sudo chmod 774 /Library/Desktop\ Pictures/Plants
sudo chmod 774 /Library/Desktop\ Pictures/Solid\ Colors
Sometimes Mac permissions require a little bit of forceful massaging. :P
Also I just notice you said Lion. If there are any other directories in /Library/Desktop\ Pictures/ that I didn't mention in the chmod 774 block above do them in the same fashion.