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It works best if you know the resolution that your monitor is displaying at.
My Macbook Pro is a 13" screen that is 16:10 and it is 1280 x 800. So when I look for wallpapers I make sure they are in 1280 x 800 so it is the proper aspect ratio.
Also, you can get a larger one keeping the same aspect ratio. Instead of 1280x800 I could have double that as 2560x1600.