Can you use pre-existing art assets made by yourself or begin creating art assets before the Jam starts?

No, unless the assets are made public online for anyone to use. The aim with asset rules is to stop asset exclusivity, so if you make your own assets and don't post them online it's pretty much the same as paid assets in how accessible they are

Ok. Thanks for responding!

If we do make our own art assets, where should we post them publicly in order to be able to use them for this jam? Is it enough to share them on a tumblr page?