What kind of images sell best?
What does Istockphoto need?
The trick to making money on these sites is to take simple images that are clear on what they want to transmit, put a whole load of tags on the images, put out lots of images and do it on as many micro stock photography sites as you can.
You should try shutterstock, fotolia, Dreamstime, stockxpert, 123RF, bigstockphoto, crestock and snapvillage.
And finally, what Yuri Arcurs recomends you to do:
It's a very good guide by the best stock photographer in the world.