My DH has done this a bit Shutterstock, which also owns Big Stock Photo, is supposed to be a good way to get your foot in the door. http://submit.shutterstock.com/ Your pics are awesome, I say go for it!
Oooh GL Julie!! I'm with Leslie..if I was in the market for good poochy stock shots yours would be SO attractive to me. I love the photos of your dogs! They are truly fabulous!