So generally in game jam everything, including art, music, code, ect, needs to be created during the jam submission period. However, sometimes you are allowed to use premade art, music, code, ect as long as you make it clear. I personally don't mind if you use some premade assets as long as you make it very clear that those were created before the jam.
I will be hosting another game jam once I hit 200 subscribers and so on, and were already at 161! Expect a new De Space Jam soon!