DALE WILSON, DIGITAL PHOTOGRAPHY SCHOOL:
"Photographer A makes an image available to his contracted agency – let’s call them Agency X. Agency X then places that image in the redistribution circuit with Agency Y. (Photographer A usually does not know who Agency Y is or what the financial arrangement between Agency X and Agency Y might be.) Agency Y licenses the Royalty Free image to a client for $10.00 USD. Agency Y then remits $5.00 to Agency X (assuming their split if 50%), and agency X then redistributes its 20% obligation amounting to $1.00USD to Photographer A."
Take a hard look at the financial aspects of going into stock photography, here.