A friend recently sent a link to a photo contest run by the Park Service. It seemed interesting and I was considering entering. Unfortunately when I checked their 'rules' (as I now always do carefully), there was a deal breaker - which I've copied below for your interest. Given this, I won’t participate since doing so is effectively giving the images away (even if I don't win anything).
“6. LICENSE/USE OF ENTRIES: By submitting an Entry, entrant hereby grant to Sponsor a royalty free and fully paid up, irrevocable, worldwide perpetual license to edit, publish, use, adapt, exploit, and modify any Entry (and all elements embodied therein), including, without limitation the images, names, likenesses of any persons or locations embodied therein, online, in print, film, television, or in any other media now known or hereafter devised, without compensation, notification or permission from entrant, anyone appearing in the Entry, or any third party, in any and all media now known or hereafter devised, including on Sponsor’s website, http://nationalparks.org, and third party websites.”
So be very careful to read their rules – and not just about the photographer retaining all copyright privileges, but what rights they have to use your image and for what consideration (if any).
Some contests limit their ‘taking’ of images to those who have won significant cash prizes, which you might be okay with, while many require the right to use winning entries for publicity/promotional purposes related to the contest.
Whatever the terms, just be sure you read them carefully (given all the legal jargon) before you upload any files (or pay any $$).
There are wonderful contests that allow you to get you images in front of large, new audiences (and maybe a nice prize, too). Unfortunately, there are also too many that are rip-offs and need to be avoided. And it takes vigilance to be sure you can tell the difference.