I think I figured it out now, so never mind, but thank you anyway.
...actually it was really simple. Don't know why this didn't work before, must've been something wrong with the pic I used then.
Anyway, all you need to do for virtually unlimited original stages is save them as i00.png, i01.png, i02.png, i03.png, i04.png and so on and as long as all of them have the correct (512x256) resolution and a low bit-depth it seems like they'll work. Apparently there's no license data to begin with.
I wonder if maybe Namco just gave us these two backgrounds to point out that we can easily use our own as well? (Well, for the slow people like me who didn't know already that is... XD)