Most people who are considering whether or not to upload to microstock sites are curious how much money can be made by doing so. (There are other reasons, too, but this is a biggie). The short answer is that it depends on how much time and energy you put into it, and it depends on the quality of your images. The top earners can make hundreds of dollars per day, but most people will not achieve that. For an overview of my experience, see below. I update the stats about once every month or two so you can get a sense of what I have earned -- without devoting a ton of time to it, to be honest.

I've written a short introductory article for eHow entitled "How to Make Money With Your Digital Camera (Selling Microstock)" -- take a look here. It is a short overview and I'm interested in any feedback you may have -- either to improve this article or for ideas for future articles you may be interested in.

Excerpt:

MacWorld has seven tips for would-be microstockers (see below). It's always helpful to read other people's summaries of the things that they find important. This is a decent list of basics.

1. Understand how it works: "...Selling stock photos is not easy money. It will take time to build up a decent sized portfolio (at least 100 pictures) before you see any sizable return..."
2. Think like a designer: "...Designers want photographs with ample amounts of uncluttered negative space so make sure you leave room for a headline or other text..." [Click the title above for more]

Shutterstock and Dreamstime allow you to sell images on an editorial license, meaning that purchasers will only be able to use the images in editorial formats (e.g. magazines, newspapers, etc). This is useful for images that you have taken which are of specific interest (famous people, places, current events...) and may not have model or property releases.

This sample editorial image from my collection was taken on the day of George W. Bush's inauguration and depicts items that might not normally have passed as royalty-free for various reasons. It is usable as editorial imagery because of the public nature of the event and the public interest in the event.

Dreamstime offers an editorial license in addition to their other licenses:

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.

Sell Your Digital Images: Dreamstime pays $0.50 to $1.00 per basic download to the photographer (50%) and has a pretty good upload system. 

Buy Digital Images:
Dreamstime charges more for photos that have been download 100 times or
more, and again if they’ve been downloaded 500 times or more. They have also recently implemented a system where print and web images are separate purchases. You can currently download images for less than $1.

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.

It's interesting to look back on the microstock experiment I started a few years ago. I have been quite lazy this past year about uploading anything--and expected my sales to dry up because of it.

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.