How to stop desktop wallpaper from reverting to default?

I can't seem to fix this issue where my wallpaper keeps reverting to that image of stars and such after my iMac wakes up or after I log out and back in.

I'm on OSX 10.5.8

Never had this problem on Snow Leopard.

Googled and found some answers which rever to a file in /library/preferences called... com.apple.desktop.plist

I looked in that folder and couldn't find that particular file.

Anyone have a simple fix?

Thanks.