I have found that the best way to manage your backgrounds is to grab the image or images you want to use, and make them into one massive file that spans the full width/height you need, then select it as the BG and use the 'tile' mode. it will spread it at normal aspect across as much screen as you have.
for instance- i use the TH2Go in 5040 x 1050, and have another 1680 x 1050 monitor to the right of them, and a little lower due to my desk setup. So i made a background file (in photoshop, but any free tool will work just as well, even web based ones like Pixlr) that is 6800 x 1300. It flows across all 4 monitors at the correct aspect, and lines up properly accounting for the dip in height on the right one...
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