Y'know, I'm seeing a lot of questions about the rules to this scattered around various forums and blogs that are signal-boosting for you. I answered the more obvious ones in a few spots but there's too many to catch everything, even copy-pasting. You might want to do a follow up FAQ for this to increase the number of entries you get. Main points of confusion seem to be exactly what you're looking for as entries, how much they can win, and whether multiple entries are allowed.
I haven't seen anyone ask yet, but more business savvy artists eventually will bring up that bit about "right to first publish" a prize-winning entry not having an expiration date, so you might as well deal with that proactively. It's pretty clear that this is partially a collection drive for random art to use in Fight On! (this is an observation, not a criticism) and that the winners - particularly the multiple third prize winners and anything suitable for an issue cover - are going to be published over multiple future issues, which means that an artist's ability to (say) sell that art for publication elsewhere is tied up indefinitely. You could address that concern by either putting a firm cutoff on the "first use" clause while retaining the use rights in perpetuity - something like a year or even two is probably reasonable given that zines don't come out very fast. Or you could just stick to it as-is, which might cost you a few entries from pro and semi-pro artists who don't want their work tied up until you use it. But say something about it, one way or the other.