There exists dozens of stock photography websites offering royalty free pictures at no cost but the Corbis advantage is that they have tie ups with big news agencies like Reuters so you could gain "legal access" to photographs that are otherwise extremely expensive.
According to Reuters, you can download Corbis pictures from PicApp website (currently in private beta) and they will be tracked automatically. The photos will either include an ad overlay on part of the image, or embedded advertising that pops up when you move the cursor over the picture.
PicApp follows a CPC based model and shares revenue based on the number of clicks made my visitors on the image but it is not clear if Corbis has any plans to share advertising revenue with publishing sites.
Corbis is privately owned by the same person who started Microsoft - Bill Gates.