I believe the use of assets from another game you made or assets made prior to this game jam is allowed. It's stated here in the FAQ in the Overview tab that, "I have code and assets from other Jams I wish to use, are these allowed? Yes! If you have the rights to use it, anything that wasn't made specifically for this particular jam is allowed. Mini Jam isn't strict on the "build it from scratch" ruling, but it is encouraged.", leading me to believe you can create the assets ahead of time.