Hmm, maybe the FAQ isn't worded very well here. So basically that's referring to existing assets you didn't make, like ones from opengameart, etc, hence the note on crediting sources. So, I'd say it's at least not in the spirit of things to make all your own art ahead of time. You can obviously try things out to see what works, if needed. But keep in mind that most people will be doing all their work within the jam period.