Rule 3 of the jam rules: “3. Please don’t use generative AI or assets that don’t belong to you. You can use any asset packs that are free or ones that you have the right to use.” So if you find a free/public domain asset pack, or if you bought an asset pack and have the license to use it, then that’s fine. But don’t reuse content from other peoples’/companies’ games unless you’re absolutely positive it’s legal (e.g. the game uses asset packs itself; even still, you should download the assets from the original source.)
Examples of asset resources/sites that would be allowed include Kenney Assets, Pixabay, or most assets packs available on this page: https://itch.io/game-assets Examples of assets that would not be allowed include music you ripped from your favorite game, sprite rips of an NES game from Spriter’s Resource (not endorsed), sound effects ripped from Skyrim etc.