I've been selling images on 4 microstock sites (including Shutterstock and iStock) for the past few years.
I signed up on these sites mainly for fun, with zero intention of shooting photos strictly to sell. Basically, if I'm out taking photos for myself, and I think that some of them might sell, I'll upload them. I'm VERY casual about submitting photos - I only have about 200 photos for sale on each site, and I started 2-3 years ago.
Here is what I've learned:
1) If you're like me, you're not going to get rich doing this. However, I DO believe that it is possible to make a decent supplemental income with microstock, if you're willing to shoot and submit a large quantity of quality photos. For myself, with only about 200 images submitted over the past few years, I have been able to pay for a good deal of my gear with microstock income.
2) If you're out shooting images anyway, it isn't much work to submit some of your best ones. All of the stock sites will read the IPTC data directly from your jpegs, so the only extra "work" you need to do is add a title & some keywords to your images in Photoshop. I spend under a minute per image adding this info. I consider the small time investment of submitting my images worth the return.
3) Most microstock sites will reject nearly everything if your images have even a hint of noise in them. Either get familiar with something like NeatImage to clean them up, or don't both submitting noisy images.
4) I shoot mainly nature & wildlife pics. I can tell you that for the most part, nature pics (especially flowers) do not sell - everyone has a million of these. Wildlife is a bit better, and I do have a few decent sellers of animals. Pictures of people (assuming that they aren't snapshots, and you can get signed releases) seem to do the best.
5) Shutterstock and iStockphoto sell the most for me. They also seem to be the pickiest about what images they will accept.
6) You don't need to be a pro to get your images accepted (I'm certainly not!). I was fortunate enough to have my initial application accepted at all 4 sites, but only barely at Shutterstock (they "rejected" 3 of my 10 images, which is the maximum before they reject your application). If you get rejected, post some of your rejections in the forums at that site (with 100% crops) to get some friendly feedback on what to fix when you submit next time.
You can see an example of some of the stuff I shoot here: http://billkennedy.smugmug.com
Everything on my smugmug site was taken with a Canon 20D. The zoo pics were mostly with the 100-400L, and the landscapes were mostly with the 17-40L.
For those that are curious, my best-selling microstock image is the orange seahorse in my "aquariums" gallery. I've made about $500 on that image alone, which is far more than my #2 image. It's strange what sells.
Hope that's helpful to anyone considering microstock.
