I am unsure as to the problem.

• You agreed to do an event
• You did the event and decided how much time to devote to it
• You knew you had no control over the final product

So, what's the issue? That they didn't choose more of your pictures? Did you ask any questions? Were the photographers told how many pics to submit? Did you ask how many they needed? Did you ask how important these pictures were?

Honestly, it sounds like you cared far more than anyone else about this project. The non-profit people were likely bombarded with a thousand pics and picked two hundred of them. Who, really, is going to sort through them for an online gallery that, I presume, had pretty limited appeal for anyone other than the parents of these kids.