Agencies like Getty's, Corbis, Shutterstock, Dreamstime and so on represent thousands of artists.
It is possible that you might also be represented by them. They have big budgets to market, and they pay that with a commission on your photography ranging from 20 to 80 percent.
They count on you to provide them with pictures that they can sell, not for your profit but for their profit. Micro sites also count on you providing them with pictures that you can sell. However, they offer you such a small profit that you never reach the minimum payout.
They only want to promote you when it is profitable for them. Only a handfull of photographers can live with the earnings they get from agency sales nowadays. When I see that, for example, Yuri Arcurs, one of the world's most successful micro site photographers, with millions of sales every year, is preparing his own website to sell directly to the customer, I think the time has come for the individual skilled photographers to take his or her rights into their own hands.
An agency has a big budget. However, when we all pay a small amount, the total is also big. When we are, for example, 600 photographers (1% of Photoshelter), which is a very small figure when you see, Shutterstock, for example, with more than 200.000 photographers, and we all pay $360 per annum ($30/month), we have a marketing budget of $216.000.
I believe that this is more than any of us can spend on marketing individually.
When your work is represented in an agency it is better to stand out from the crowd. It is important that a potential customer is searching to you, otherwise the chance to sell something is a lottery. However, when there are blogs linking to your pictures at your agencies, those pictures will be seen ahead of others without this feature.