Winding road wallpaper: http://res1.windows.microsoft.com/resbox/en/windows/main/fc02eae6-095b-4e18-82aa-f37c9706f6b3_6.jpg. 3840 x 1200 pixels dual-screen wallpaper.
beautiful waterfall wallpaper: http://res2.windows.microsoft.com/resbox/en/windows/main/324425b2-0405-4b0a-841a-f626bc36aa29_11.jpg.
Tsing Ma Bridge (Hong Kong): http://res1.windows.microsoft.com/resbox/en/windows/main/73d5c047-18ac-4a85-83e3-370a2f4f5e80_6.jpg. 3840 x 1200 pixels dual-screen wallpaper.
Cliffs at Sunset, Muriwai Regional Park (New Zealand): http://res2.windows.microsoft.com/resbox/en/windows/main/c9419d5a-dff2-42a0-9a35-fef4cd7d7adc_6.jpg. 3840 x 1200 pixels dual-screen wallpaper.
Some more wallpaper examples in an older posting: http://www.securitronlinux.com/bejiitaswrath/awesome-dual-and-quad-monitor-wallpapers-suitable-for-a-linux-desktop-environment/.
An awesome quad-screen wallpaper looking into the sun. This is an awesome panoramic image: http://farm2.static.flickr.com/1324/870228536_04429559eb_o.jpg. 7351 x 1896 pixels quad-screen wallpaper.
A cool planets panoramic wallpaper: http://www.wallpaperpimper.com/wallpaper/Quad_Screen/Planet-Scape-5120×1024.jpg. 5120 x 1024 pixels.
This is a massive wallpaper for a large panoramic setup: http://parkerlab.bio.uci.edu/pictures/photography%20pictures/2012_09_17_AFRICA_SELECT/CapeTown_PANO.jpg. 11803 x 3079 pixels.
A lovely Unreal 3 wallpaper for quad screens: http://www.quadmonitorbackgrounds.com/albums/gaming/unreal3deimos.jpg. 7680 x 1200 pixels.
There is also a list of some lovely GTK themes and wallpapers here: http://www.securitronlinux.com/bejiitaswrath/lovely-gtk-themes-for-the-linux-mint-mate-desktop/.