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. The nice thing is that even the images you uploaded first remain in play, and as your library grows substantially, so do your earnings. Each time one of your new images comes up in front of a designer's eyes,there is also the possibility that she will either purchase it or click to view your portfolio. I haven't done very well with this and plan to try uploading a few images at a time more regularly, instead of a big cluster at one time.

This sample image is one of my popular pictures on Shutterstock. It qualifies as royalty free because there are no identifiable people without model releases, no logos, and no other imagery which would disqualify me from selling it.