At conceptart.org there are many of similar daily and weekly and monthly challenges, they are run by moderators.
Normally a basic principal is established and then everyone is invited to make topic suggestions, the list is collated and a topic is dished out by a moderator.
Voting systems are also used (if memory serves) to help pick winners.
Now that people can post videos easily, this could work well....... thinking.
hhhmmmmm.
![Cool 8)](./images/smilies/icon_cool.gif)