I would rather suggest you to buy assets.Many already would had/have been using open source assets,so it gives a very bad taste.Also using open source assets depict less seriousness of the programmer towards game development.
That doesn't really work in the game jam scene.
The Idea is that if you come up with something worthwhile you then replace the artwork if necessary.
There's more than enough free stuff out there to make something that looks decent.
Besides everyone knows if you're realllly serious you make your own assets from scratch.