when you upload the picture (or after you can edit it) you'll find a Tag section. write what you want for exemple the sunset contest yoy want to be... hope it helps, sorry i'm not native english langage ;-)
to add to Helene's... each contest has a tag and when you upload your photo to your page, you tag it with the appropriate name. the judges for that specific contest will look at all the photos with that tag and declare finalists/winners.