Okay, it depends on the type of pack. If you are releasing a pack of modular tiles that can be used to create dungeons then they should just be a zip of pngs or jpgs. If you are creating a single map that you want to be useful in maptool and other vtts then you can create a .cmpgn file with the map in (remember to get the distances right for 4E and 3.5/PF). I personally add in the light blocking geometry (VBL) and place some basic light sources (remember light sources differ for 4E - square - and everything else - round). Other vts won't be able to use maptool .cmpgn files so you also need to add in the unadulterated map as a jpg. I also add in a multi-page pdf of the map that people can print out at the gametable and use. This is best created in A4, and US letter.
If you can spare $2, pick up one of the map packs that I've done and you'll be able to see how I've tackled the different problems. I'm certainly not going to claim this is the best way to structure a map pack, but I've built up this format using the feedback from comments over the last 6 months or so. I'd suggest looking at the most recent:
The Low River Map Pack
And .cmpgn files are just used to store adventures. This can include a rules system, but doesn't need to.