Stipple gets its images from multiple sources and one such source are the various stock photo agencies that we work with. Since these are high quality images that are in demand, the agencies have asked us to put up a nominal fee to prevent abuse of their precious asset.
As a publisher, you license the right to use a Stipple'd image on your site. You recover the license fee (depending on your site traffic) from the amount you make via that same image as a Stipple Publisher.
You are charged for each image and the amount shown for each image is the amount you will get charged, nothing more. For instance, if you like two images and one of them is $5 and the other $25, then you will be charged a total of $30 (one time fee) to download and license those images.