You must own the rights to any photograph submitted for the photo contest. This advisory has been written into some monthly contest announcements but it should apply to every contest. Usually this will mean that you took the photo entered in the contest. At the time you enter a photo it's only fair to the other entrants to say if you didn't take the picture but that you own the rights.
From the Community Rules at the bottom of each and every Forums page (underline mine):
Do not post protected / copyrighted content:
Information copyrighted or owned by any individual or entity other than the member should not be posted on the discussion forums without the consent of the owner.
At AIR Forums you may see duplication of advertising copy, manuals or web content. These generally are used for discussion purposes. Such images could be removed on complaint of the rights holder.
The mod team