All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

It's annoying that the forum kicks you out when you're inactive (collecting screenshots).

That's strange: I've never had that happen. Well, if that's the case, prepare everything in advance, and then go to the forum and immediately paste it into the message.

What does "main hero" mean? The one you specify in the losing conditions?

The main hero is usually (but not always) the strongest warrior, the one who captures new castles, objects, and fights enemies... Usually, again, he is defined by the mapmaker at the start, and the player recruits the rest "to taste".