As with everything, securing your work against theft is a matter of evaluating the risks and taking reasonable measures to guard yourself. Who wants it? What measures are they likely to take to get it? How much do you want to inconvenience yourself to protect it?
In the case of these maps, the major "threat" comes from people who gather large collections and can't be bothered to track where the images come from. Then they post them all on another site without attribution. Something as simple as putting a visible signature on the image, preferably with a link to your own website or deviantArt page, will protect the image. If you really think there's a risk that someone will try to pass your work off as their own, then the best solution is to keep that work under wraps. That sort of thing is quite rare, though; I am not aware that it has ever happened to someone here. It helps that this is still a relatively small community and fantasy mapmaking in general is a very specialized niche. There aren't many places where a map could be posted that one of us won't eventually see it, and if it's not in one of those places, then its exposure will be so low as to be immaterial.
So the big question to me is: How much do I stand to gain by posting here? Nothing is as beneficial to a piece of art as a good critique. I consider the risk of theft to be quite low and the advantages to be quite good.
If you're still concerned, though, consider posting just a segment of the map—enough so that we can see the style and technique so we can give you some feedback, but not so much as to compromise your ultimate use of the map should that segment get out of control.