There's a ton of great public domain work, and it's encouraged for this jam! I'd say if you're commissioning work, be sure you have commercial rights if you're selling it! It's one of those cases where you're best communicating with your artist, or sticking to public domain art with good faith looks at using licensing.
That said, your work doesn't need to have art at all, and a lot of my favorite games are plaintext documents. Art helps communicate vibes, but it's not the only way to do so. I'll try to compile some resources for good public domain options, but submissions aren't graded, so just have fun and make something cool. Nobody will slight you for not being a master graphic designer.