Dreamstime is one of the top tier microstock agencies and the top 3 reasons to stay as a contributor are written as follows:
1.High commissions. "Microstockdiaries" wrote about it: "Their commission for photographers is 50%, among the highest in the market, and certainly the highest of the top tier microstock agencies". In fact it's from 50% to 80%
2.Leveled value for images. There are 5 levels an image can reach when downloaded often... The more it gets downloaded the bigger the level... and the bigger the level the more it pays you.
3.Assignments. Every month dreamstime puts up an assignment - a great opportunity to display your images on the dreamstime home page. You increase the exposure of your images, raise their level to top level (for images accepted in the competition) and eventually win an iPod (the usual prize but some assignments prize you with $1000) if your image gets the most votes.