So, you want to be a stock photographer ...

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.