I'm another one who changes wallpaper (whether it's computers or cell phones) with the seasons, for the most part. I recently had a very nice (but too bleak after a few days) late Autumn scene, after the leaves were gone and with just a little ice on the trees. It turned out to be too bleak (even though it was beautiful), so I switched to "pre-Christmas/pre-Winter" thing with green leaves, a few berries, and a little snow. I have twin wallpapers on my laptop and desktop (for my own sense of stability and "mental consistency" )
Cell phones: depends on the phone and on what kind of screen it has, but generally, it's always something seasonal that involves flowers somehow. I take my own photos for wallpaper lots of times.