The stock houses have certainly comodified the sale of photography and just about every other two dimensional media, paintings, graphics, etc. You can lisence AA images on Corbis. Muench also has images with them as well. You can buy fine art prints from famous artists who are long gone at huge discounts.
There are 10's of millions of images stored in these houses, maybe 100's of thousands of artists. There are also smaller agencies that sell and market images of the larger ones. This all leads to smaller commissions for the photographer/artist.
They can certainly control trends in what gets marketed to the masses, as to what the masses see and influence their thinking, public perception, and most important, how they spend their money. I am sure they also have a powerful influence on the spread of digital.