Every time someone does publish with you they are setting themselves back because they are not growing their own portfolio of apps but instead enlarging your network at significant cost to themselves.
I'm not going to go through with this either way - this seems like a good deal for those that really don't have their feet wet with games or creating their own art. I graduated with a BA in art (graphic design), so I don't really trust a whole lot of people when it comes to them creating the graphics for my own products. They'd have to be damn good. No offense to Make Games With Us, but I could handle my own on this one for sure. Hopefully you get some good contestants and creative ideas though. Can't wait to see how it all turns out.
In any case the issue of iStock art took away from my main point and that is publishing with you guys gives away the most important ingredient of becoming successful as an independent developer in the app store. That is the network effect of having a bunch of your games installed on peoples phones and computers. When someone publishes with you they are trading a potential short term boost in sales for a long term disadvantage in the market while at the same time growing makegameswithus own network and brand instead of your own. As an indie dev it's a foolish tradeoff to make if you truly want to have a sustainable app business and not get squeezed by yet another third party.