When entering photo competitions (or when uploading images to websites), you should always find and read the fine print to see what rights you have just granted. In general, responsible photo contests should only have rights to use your images in conjunction with the contest itself (e.g. announcement of winners in publications and online, for promotion of future contests, etc.). But many contests, especially those run by larger organizations, state that they can use any entry however they want, in perpetuity. In most cases, the language is boilerplate and may not be something an organization intends to take advantage of; Regardless, you should only enter contests and upload images if you are comfortable with the language.
Those of you who are more into dramatic recreations might want to click through to read our screen play about a fictional negotiation between Promotions Person and Mr. Lawyer, written by Wetpixel moderator TheRealDrew.