I did a quick analysis of earnings from ShutterSTock over the past 2.25 years:
Clearly, the bulk of earnings come in drips and drops, those tiny "25 a Day" sales (which used to be $0.25 per image, but now are $0.33). I've only had a couple of Enhanced sales, but they add up quickly (at $28.00 per). The On Demand sales are kind of new, it will be interesting to see how much they increase in ratio with time...
Not sure how other, more dedicated, microstockers measure up, but I'm going to ask at the Yahoo group...
Happenstance: “Chance or a chance situation, especially one producing a good result” ... Photography: "the art or process of producing images on a sensitized surface by the action of radiant energy and especially light"