The BoundingSquirrel Nature Art Wallpapers App allows you to set Bounding Squirrel digital paintings as wallpaper (ie the background on your Android phone or tablet) for free.
The paintings are lower resolution than the real things, but for reasons to do with screens, this makes no difference when you're displaying them digitally.
If you want to grab the wallpapers then click here, or on the Google Play icon, below.
If you're an Apple or Windows user, please bear with me.
It seemed easiest to start with the tech with which I'm most familiar...