When we ran our Photo Contest a couple of years ago [5 years to be exact], one of the Terms and Conditions that we had was that a photograph would remain the entrant’s property but they give us the right to use it for a reasonable amount of time (and we said one year) for the express purpose (and nothing else) of advertising the contest for the following year.
You’ll see other responsible contests stipulate something similar. [Of course, we copied them, and not the other way round.] And that’s fair. You get the exposure and we get to show how great the contest is. After that year, we do not have the right to use your picture at all in any way except, of course, by common agreement between the two parties. [By the way, running a contest is very taxing and we haven’t recovered from the last one yet ;)]
What you do want to be aware (and beware) of is when contests stipulate that they retain the right to use your entry “for perpetuity” without remuneration. It seems this lingo is in many of the “amateur” photo contests because the sponsors just don’t want to bother getting your permission. It covers everything. Some pseudo contests just want to get great photos and videos cheap. Knowledgeable photographers tend to avoid these contests, not wanting their photos to be used however and whenever a company wants to use it.
Whether you choose to enter these contests is up tp you. If you do choose to enter such contests, do read the fine prints carefully and be fully aware of what you are signing away.
Here’s a link for further reading on contests.