Twelve South are a brand well known to Mac users for their laptop and iPhone/iPad stands amongst other products.
While browsing their site tonight I came across this page of wallpapers specifically designed for dual screen setups. There are wallpapers in resolutions from 11" MacBook right up to 5K Retina iMacs. There are 8 different sets, 4 of Scottish landscapes and 4 of San Francisco skylines.
Worth taking a peek if you are looking for high quality wallpapers to show off your dual screen setup ( or even just for a single screen setup… )