Creating your own hero.

I read it, but I still don't quite understand. So, you can create your own hero based on an existing one, change his picture (I think it's better to do it with Paint.net), and modify a bunch of other parameters. You can make him look like any monster, etc. BUT! To use him in a map that you didn't create, or to play online, you need some kind of program. Is that right?
And another question: will the hero I created be available when selecting a specific map on which I created him, or will he be available on any map?
If anyone has the time, please tell me what I misunderstood. Thanks in advance!