I always just use googles image search. Just set it to large pictures. Might aswell just add "hq" after the search term so you will most likely get some larger pictures.
I got the interface lift one from one of your posts in this thread and really liked it because it had widescreen wallpapers, as I used to use 1024x768 pictures and stretch them, doesn't look bad for 3D art stuff or land scape stuff if you do that, but you can't use any other type of wallpaper (such as cars or people).