Here is a selection of photographs available to download directly for use as a PC background image. They are 1920x1080 so will look beautiful on your desktop no matter what screen you use. You may select a single image for £1 or buy as many as you like (even the entire collection) for just £5! 90% of your payment goes directly to charity.
Tip: why not buy all images and set your background to a slideshow so that it changes periodically. Instructions on how to do this can be found here (windows)
or here (Mac)