May be you should try a different spin on your business model. Instead of trying to setup one big central site and attract photographers to it, you should creat

May be you should try a different spin on your business model. Instead of trying to setup one big central site and attract photographers to it, you should create a product that can be hosted or installed on photographer's own site. George Lepp was mentioned earlier in the comments. He set up sales directly from his web site and does not have to pay to stock agencies. I bet many photographers would like to do the same - a niche your platform / product could fill. It should be beautiful out of the box, yet customizable since photographers are artist and sure they will want their own look and feel. It should offer easy integration with payment systems (major point of pain), help with discoverability (built-in SEO), embed watermarks for free demos and what not (you should know that way better than me). In other words don't try to be next Amazon, Zappos, eBay or Yahoo Shopping of stock photos. Be shopify.com of stock photos. Different model, different niche, maybe even different requirements to the product, but still something to consider. Good luck with your endeavors!