A New "Business?"

I recently received a query from a book publisher, inquiring about the possible purchase of a photo I have up on one of the free photo storage sites. Now I'm checking out how to sell, how much to sell for etc. I'm in the dark totally.

As I googled it all, I came across site/businesses which sell "stock photos." Do people really make money like this? Are they reliable?

HELP!

Thanks!

ps I just wrote to the publisher to check if the person who wrote to me really is their art editor, since the email isn't from the publisher.