1/ Images are like art. Taking preferences of buyer and seller into account, they preferably sell only once (or as few times as possible). No buyer wants that image to appear in similar publications and so every transaction is unique. Super-stores, however, are modeled to provide one-to-many sales transactions and are therefore NOT suited to support the image exchange marketplace.
2/ Except when images are produced on a commissioned photography basis (for example by Getty-Images staff photographers), the image super store actually does not own the image, it merely has a right to operate as a reseller. Nothing would stop a photographer from trading his images somewhere else, dramatically deflating the value of the super-store.
Fact remains that $22B of images are exchanged every year, most of it (90%) not through online transactions or the sum of all super-stores. This represents a big opportunity not many Venture Capitalists understand, as it is a market-play rather than a pure technology-play. But established companies may be able to build an iTunes of images to feed their ecosystem of products.
In the meantime, Getty-Images (now private again) keeps on puffing itself up like a puffer-fish. The question is: how long will it be able to hold its breath.