See Wallpaper, Logos, & Art on Pscience5 for more sample pictures, and some instructions for making your own.
These have all been faded to a lesser or greater extent so they don't overpower the Desktop/System file listings.
This one is 545×454, which I found to be the best fit without any cropping in any of the various toolbar and title settings. This was important as the white background of the picture goes seamlessly with the white border enforced by the Desktop/System screen. She's stuck off to the right to reduce the interference with the Desktop/System file listings.
Desert Wings by Keith Parkinson:
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These are the wallpaper images that have graced the background of my MC218 at
one time or another. All of them link to an EPOC bitmap file which you copy into
your \System\Wallpaper
directory (create the folders if they don't
already exist).
I have found I get best results when I create a picture without any black in it. This ensures you should always be able to see the black text in front of it. The second Tomb Raider wallpaper below is a good example.
The maximum size your graphic can be without ever being cropped by the operating system (taking into account the toolbar and title bar) is 537×208 for Series 5, 5mx, and MC218 machines, and 537×448 for Series 7 and netBook machines. I don't have a Revo, so I can't check the dimensions for it, but they should be close to 377×128.