The rules are pretty open.
The photo should be taken by you. Any modification to the image should only be done by you, and finally the picture you use should be one you've taken specifically for the competition. No old photos please. Part of the challenge is receiving a theme and then working with it. The photo can be manipulated in any graphic arts program of your choosing. Using availalbe effects and things of that sort to enhance a photo is welcome. Blending bits and pieces of other images into your photo is frowned upon. For example, if the subject is "water" and you photoshop a body of water into a photo that didn't have one, you weren't really taking a photo of water were you?