I promise, my blog is not going to turn into a "desktop wallpaper collection", but this one is too cool not to show it to you:
"Ceci n'est pas le fond d'écran officiel de Firefox / This Is Not The Official Firefox Background Picture"
By the way, the photographer Denis-Carl Robidoux publishes a picture per day, all of which in different sizes to fit your screen, and for free, too.