I'd agree with most things blakeb1 mentioned. One then I'd like to say though is, it is important to stick with things you know (like blake mentioned), but you can also turn a Jam into an opportunity to learn something small! The reason being is, Jams push you to make something and push you out of your comfort zone. This is a fantastic reason to learn something! Maybe you think you want to make a game like flappy bird. Well then you'd have to decide: "This is a pretty small game, so does this give me time to make my own art??" And for a jam like this i would say yes! So this gives you a reason to learn pixel art!
Hopefully that made sense and good luck on the jam!