It is truly sad that an organization the likes of the UN (so seemingly dedicated to the rights of peoples across the world) would sponsor and promote a photographic competition that exercises the option of using photographers' images at will- without payment and/or credit. Certainly you can fathom the inherent irony that such policies promote.
Poverty is all about the ability to ignore the plight of peoples deemed "expendable." Having people fairly compensated for their labor is one of surest ways to end poverty worldwide. Your policy of appropriating the images of those who wish to participate in your worthy endeavor certainly does little to reinforce, and in fact undermines, both the spirit and the overall credibility of your stated goal. Kindly take the lead and set the example.