This article is nothing more than an advertisement. The company that makes freelanceswitch also owns the sites listed: FlashDen, AudioJungle, ThemeForest, iStockPhoto
I think the issue is that AFAIK it is not that easy to make $2000/y with selling stock photography. Actually, it is a hard, persistent work. So he was propably just lucky, so they picked him up to advertise them.
I'm probably not enough of a graphic designer or artist in general to do exactly that, but this post made me realize how I should really think of ways to make some money on the side.
And also that, just trying can sometimes be enough!
What would you see as the closest thing to stock photos for a programmer? You can sell your software of course, but it feels like it needs a lot more investment and that a few hours at night on one small bit of software won't make it.