Good Afternoon, Rebecca,
I worked with a small St. Louis stock agency for several years in the 1980's and early 1990's. Obviously, I'm out of date on some current practices, since digital was only in its early stages when I became too busy otherwise to continue in the field. I'm sure, though, that Baxter's advice above is on the mark. Earnings are unpredictable and rarely enough to constitute one's primary income. The number of images required to lead to even a few sales per month can be quite large. I was satisfied with the agency I worked with because I never expected to make a great deal of money; as a teacher, I had some free time during summer months and could produce enough images to make an occasional sale--enough to cover costs and buy a new lens or some other piece of equipment from time to time. It was an interesting and pleasant experience, but, when the agency was sold to a larger one, I decided to give it up and just shoot for fun.
Konical