Two websites offer more than ten thousand ultra-high resolution wallpapers for free (up to 2560x1600).

These two websites offer tens of thousands of free ultra-high resolution wallpapers (nowadays, many high-resolution wallpapers online are paid), with content ranging from hand-drawn to photography, all very comprehensive. Most importantly, they are free.

http://www.caedes.net/

http://interfacelift.com/wallpaper