The contest is for spots that are exactly 35 seconds long (“entries shorter than thirty-four (:34) seconds and entries longer than thirty-six (:36) seconds in length will be disqualified”)
ABC is providing images, audio, and video clips that will be fair game. You can also use your own materials, provided you own the rights to them. The grand prize? Your promo will air during an episode of the show.
http://abc.go.com/shows/lost/fan-promo-contest