iStock just announced that they will be selling logos soon. An excerpt from the press release is below, but first here are a few reactions (the reaction from designers is overwhelmingly negative):

Digital Arts

Shutterstock, Dreamstime, and iStock allow you to sell your vector illustrations on their site in the same manner as the digital photography. This means you can create imagery in Adobe Illustrator, Freehand, Xara Xtreme, or Inkscape (free download). You earn money for every download from their subscriber base, with additional incentives for referring buyers or sellers to the site.

This sample image is one of my most popular vector images on Shutterstock, from a series on road signs I made. I drew it in Illustrator and saved it as an EPS.

If you're an artist, create vectors--they sell well and if they are sufficiently detailed, some microstock sites will tag them as more expensive downloads--meaning more money to you. Selling vector images at Shutterstock means working within the (essentially) all-you-can-eat package--which can also mean many, many downloads.

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.