Let's make a deal... If you plan on completing your first game jam and learn from it... Then you could use free assets if you choose. Please make sure to source them. These game jams are more focused on getting everyone making everything from scratch eventually so its ok for the first time. Completing your game will be an amazing first step for you and I would rather you focus on that, instead of trying to make your art "perfect".
Note for prizes, the rules would have to be followed, so any attempt at your own art, favors much more in the originality part of judging then using assets.
We also have Practice Circles in-between our monthly Beginners Circles if you are looking for a no rules, get plays and feedback kind of jam