Here is a set of wallpapers that feature this exact illustrated-by-hand concept, though authors have not indicated that they used GIMP or other open-source image editing application. Also, some are from general wallpaper search sites, which I suspect don’t always link to the original author, but a number of these have a watermark (smart!) on them so they’re more traceable and attributable.
You can also find a few tutorials after the set of wallpapers, which may help guide you should you become inspired to create your own!
Wallpaper Collection
1. Halloween by Sandara
Download at: Deviant Art
via makeuseof.com
There are lots more!