I once managed to make my own zone picture, but have now completely forgotten how to
I think the pictures (.LVL and .FIN) are in .pcx format, but that's all I know
As for posting zones, [zone].LVL, [zone].FIN, [zone].MAP and [zone].NME are the files you need, after you start playing a zone, a [zone].LST file is created, I'd recommend that you don't post that with the others, as it lets you skip ahead to the last level played.
Thanks (DarthHelmet86) for correcting my post, I'll try to use the Offers forum to share this game, but is posting download links to self-made levels allowed?