It is, but they have to be in BMP format. If I remember correctly, you just put them in the Windows directory and they'll show up in the control panel.
Oooh, looks like somebody's got a better memory than me It's been toooo long since I've used 3.1
Edit: I couldn't resist.. I already had 3.1 installed under VMware on my laptop, and I fired it up.. turns out it is called "Control Panel" I guess my memory isn't as bad as I thought
Ive never used 3.1, Ive used Windows v 1 or 2 (half the programs ran on DOS anyway) on a "Tandy 1000RL"
when it was on the Hard Drive sounded like a bloody fire engine