PicScout Makes Image Credit Available to All for Free -- Spurs Industry-Wide ...
PR-USA.net (press release)
... by a tiered pricing structure for additional clicks based upon the image's licensing status: rights managed, royalty free and microstock images. ...

Dreamstime offers four extended licenses which could increase your earnings significantly: Increase Max Copies, Web Usage, Print Usage, and Sell the Rights. Each of these extends the ways in which the image can be used and pays a much higher premium. If you sell the rights to the image, you no longer control it and cannot sell it elsewhere, so use it with caution--noting that you can make a large chunk of change using this option (you can set your own price). Further details are below.

If you want a constant (and constantly growing) revenue stream from microstock--upload new images regularly. Many of the sites sort images in a number of views from newest to oldest by default, so that designers who are regulars can find the fresh photos and vector images quickly. This means that the more new images you keep in front of them, the more likely sales will be. As well, the more images  that you have in your library, the more statistically  likely they are to show up in keyword searches.