Here are some of my favorite sources for desktop wallpaper.
Bruceb Photos
Favorites - widescreen
Favorites - square
Favorites - dual monitor
Favorites - phone
Windows 7
Windows 7 - Russia
Hamad Darwish photos
Mike Swanson photos
Apple Snow Leopard
Windows 7 Personalization Gallery
Bing images
Interface Lift
VladStudio
Digital Nexus
Hamad Darwish
Mike Swanson
