The wallpaper images have moved
Originally, the photo galleries only contained low resolution copies of the various pictures. Now, all pictures are available in full high resolution form. This means that the wallpaper section isn't needed anymore, as everything that used to be available in it is now available through the galleries and more besides.
The main photo galleries are here. If you are interested in multi-head wallpaper for multiple monitors, you can find full resolution panoramas as well as panoramas cropped and resized for use as wallpaper over multiple monitors in the panorama section here.