You don't want to force people to do anything, otherwise it skews the results. Force someone to vote for 30 seconds, or to vote 10 times, and they'll just sit there tapping the first picture until they reach their target. You'll get lots of votes, they'll just be useless.
I think you'd get enough votes if the app was in the right format. It would have to be like hot or not, where you have 2 photos, pick one, and instantly get another 2 photos along with the results from the first set. This gets people trapped in the just one more click mindset.
Focus it on fashion and clothing, people will vote to see more photos, to judge the outfits of others (a lot of people enjoy doing this for fun), and to get inspiration for themselves.
Also, allow users to select what gender and age groups can vote on their photo.