You can definitely post a WIP on your first post, mine certainly was!

I never thought about people stealing since I haven't made anything worth stealing yet either. However, I would love if people used my maps one day for their own campaign. I think it just means I did something right
Then again, I don't intend on making any profit off my maps. As for the professional artists on the forum, my best guess is that there is some sort of honor system in place. However, theres probably also an advantage in letting people see some of the artwork to encourage them to buy the rest.