Note that we call the following “distributors.” Most are not “agents,” who are required by law to watch out for the best interests of those they represent. This is a key change in modern stock contracts. Also note that one distributor - Getty Images - now dominates the market.
Be sure to check the links listed above before investing too much of your effort or work in any distributor or licensing model. These are a rapidly changing markets, with innovations in licensing products and contracts. This is a short list of a big industry. It includes only sites where “rights-managed” images are available (although most also market royalty-free images). NPPA does not support royalty-free licensing of news images.
Photoshelter - This is a portfolio, online storage and commerce service, but it enables photographers to set up their own stock licensing (and pricing) system, alone or as a collective.