Hi Chris,
Stock photo sites are quite numerous these days. The good ones tend to have very high submission conditions for acceptance, and so be sure to read their guide lines thoroughly before sending them images. These days you can upload low res images for them to see and choose, but they will require fairly high res stuff for offering to their clients as a rule. Stock sites range from the top of the tree like Getty Images, to the smaller micro stock sites like iStock linked to here.
Overall, they are not places where you will make a fortune. Even earning a small amount isn't guaranteed. To begin earning regularly, you will need to have at least 1000 images in a bank. The really successful stock image photographers, who do it for a living, usually have their own studio setups.
Watch this video from Youtube for further guidance:
http://www.youtube.com/watch?v=-KCd9...eature=related
Two sites to consider contributing to below
http://www.istockphoto.com/sell-stock-photos.php
http://submit.shutterstock.com/
And finally this:
http://www.microstockdiaries.com/how-to-microstock.html