I have a variety of microstock accounts that earn me beer money and have paid for lenses. I tend not to bother with istockphoto too much because the upload process is a real pain and they treated their contributors like crap in the last year.
Your best bet with MS is to get as many accounts with the major agencies as possible because once you have edited your photo and keyworded it all it takes seconds to upload else where. Fotolia, shutterstock and dreamstime all do well for me, 123rf has sluggish sales but it is very easy to use. Different websites tend to do well at different types of photos, so some that never see sales in one places will get hundred at others.
Shutterstock is my fav At the moment. Sadly I avoided shutterstock like the plague because I didn't like the idea of selling photos for 25c and hated the subscription model. But over the last 5 years every agency uses e subscription model now. At shutterstock sales are far more frequent and I make far more money there than elsewhere. Also I get lots of higher value special sales at $30-50 a pop which very nice.
Saying that istockphoto has higher prices and so if you find that your photos work well there then it can make a lot of money, more so if you exclusive but in general this is a bad idea. I suggest try all major agencies for a year, once you have an idea of what sales, where, how much each agency is paying you our then you could consider going istock exclusive but for most people it is never worth it.
It is hard to say what photos sell well. Th obvious ones involve blond models in an office working at a computer or some other corporate meeting etc. boring as whelk and I don't touch that, I mostly shoot what I find interesting and I want to do == nature. This equates to not very good sales. But I don't want to shoot stuff just to make sales, MS doesn't pay enough to make that worth while.
One annoying thing you will find is some photos will sell well others not, and it is not obvious why. Some of my best sellers are a very boring scene/object, shot with no effort, hand held, with a high ISO, 5second edit, something anyone could replicate better, and yet they sells really well making hundre and hundreds of dollars. Other photo that I think are more unique, or much more interesting, that required far more work, that only professionals could replicate with time and dedication don't sell at all. My best photos get basically no sales, it is not what stock wants. So I basically don't upload them anymore, and will sale these as framed prints for a few hundred each (currently in process of setting up a website, spent weekend learning wordpress).
Tbh, the glory days of MS have ended. 5-7 years a go it was easy to make a grand a month throwing up whatever photos you took with bad key wording, poor processing and mundane subjects. Now a days you need to provide 10 the. Umber of photos, all of perfect quality and get a tenth of the sales. It really feels like it is 100x worse that when I started.
