When submitting images to a contest, be sure you read the fine print of the contest rules first. Some contests are simply scams for the contest holder to acquire unlimited rights to all contest entries for free. Regarding unsolicited magazine submissions, I would only submit watermarked, web-sized versions. There is a fine line between watermarking enough to deter ripoff and watermarking so blatantly that the image is ruined.
As to protecting the idea in the photo (subject matter, composition, etc) there isn't much you can do to stop the magazine or whatever from hiring someone else to go shoot a similar shot, other than sue after the fact.
