You can. As long as you have the right to use it and distribute it. I think submitting it to the Game jam legally counts as distributing it. There is a great FAQs and asset section where some free assets are listed.
Thank you for posting FAQ links! Quoting the relevant bits for you here:
I can’t draw/pixel/compose/etc. Can I use someone else’s work or asset packs? Are contestants allowed to use already made assets?
Yes, but only if you have the rights to it. It depends on the copyright attached to the resource you have in question. Make sure you are 100% sure a resource is free for use, or is under a Creative Commons License. If you’re not sure, it’s always safe to ask the content creator! However, your best bet is to use resources in the public domain.
