It's been an interesting amount of work for perhaps a minimum return. But I'm glad that Fotolia accepted 9 out of 28 photographs that I have submitted. Back in the days before the inter-net, I was trying to break into the world of Stock photography. In the early days of Tony Stone, before it was sold to Getty's image, submission process was cumbersome. This was in the early nineties. Photographers send in slides, then it was upgraded to high quality jpg files. Now many people are submitting there works.
Shutterstock is also another good source for micro stock: Submit Photos to Shutterstock and make $$$!
See chadmcdermott.blogspot.com for more info about micro stock.