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Checked out some of the competitions and the ones I saw did say 'You maintain the rights to all of your submissions'. Not sure if there is any other small print though. Like other posters have already mentioned: be wary of images harvesting scams under the guise of competitions. Great idea if you think about: you give away a $300 tripod and exchange get thousands of images you can then pimp out to your heart's extent.
Unless a photo contest is being held in a physical gallery with no online exposure whatsoever, most of those contests have to have what seem like kinda scary legal terms. Generally you have to give them an irrevocable and often perpetual right to display and use the images and on and on and on. If they don't put that in there, then they can probably get sued if they post images online (where someone will probably copy them), on any Facebook page where thousands of people may share them. Even Nat Geo Traveler terms seem pretty ugly. About the best someone participating can hope for is to keep the copyright, to limit image use to activities related to the contest and not agree to permit the contest holder to sell the images unless it is a rev share with the photographer.
IMHO, most reputable photo contests are not out to screw you and steal your images. Given that most any photo contest has terms that seem a bit scary, skip anything that doesn't have a pretty significant prize or offer some really good exposure unless you don't care what happens to the photo. There should be a recognizable company behind it that could be anything from National Geographic to Smithsonian to Nikon, Canon or even La Mer Skin Care products but even then look at any entry costs, competition & prize. Nat Geo Traveler is probably a HUGE money maker for Nat Geo (and a complete waste of money for any photographer who does not have an absolutely spectacular photo) at $15.00 per image with virtually nothing in terms of prizes considering the competition and the insane amount of money generated by that contest.