If you are using a machine that does not have a lot of RAM you might be able to save some resources with this tip.
Make sure your wallpaper fits your display or is as close to the actual screen size as possible. You can use the scale feature in Gimp to get your wallpaper sized. Use .jpg for wallpaper on systems that have limited memory, .png files are sharper but are larger in size. You can convert a .png to a .jpg simply by opening the file in Gimp and choosing to save it as a .jpg. On systems with limited RAM this tip can reduce memory use a little and can make the desktop load a tiny bit faster.
I tend to tweak out my machines and have done this a lot. You would be surprised at the amount of resources you can save.