You're in a bind in this kind of situation. You would probably be best to avoid stock photography completely.
Realize that you're going to be working mostly for free. Even if you win a few, the payouts in these "contests" are painfully low. Averaged across all your entries that don't win plus the few that do (even top talent on these sites runs about 1 in 8), your hourly rate will be very low indeed. I'd say your chance of winning if you use low-resolution comp images is nil, since untrained clients don't understand about low-res comps. To them, it just "doesn't look good." They also don't understand "something like this, which we'll replace with a real stock image later" if you can't sit down and talk with them.
You ask how others approach this situation. Scott and jmort253 basically told you. It doesn't arise because we don't do free work in the hope of maybe winning a few hundred dollars. Not even to "build a portfolio"; certainly not to make a living. I make a good living as a freelancer, and I've never done spec work. Nor has any other successful designer I know.
When the work is for a real client, cost items such as photography (stock or specific to the job), travel, purchase or rental of props or equipment are either worked into the bid price or are specified as a separate line item over and above the design service.