My best advice is to never post full resolution images and never post the "source" file (PSD). Unscrupulous folks can still steal it and say it's theirs but you'll win any actual legal discussion of ownership if you can produce the original files at a higher resolution than they can. To my mind, the legal issues are the only ones that really count...any "reputation" issues (i.e. some schmuck claiming your map is their work around the Interwebz) are more easily resolved and less important.
M