Hey, you have to make all the assets in the jam period. You cannot use old assets, unless they're released on the internet for free, for everyone to use.

The first Jam I ever joined was an OpenGameArt.org jam, and the idea was anything you made you had to put up.
Maybe you could upload your assets there with a permissive use licence (It has a list), and link them here, if you want to reuse them?
Might need Gurpreet to confirm tho