Image Probability is the probability that a link on a made-up page goes to an image rather than to another page. The choices run from 0 to 0.1.

A link chosen as an image points at an image.jpg under the page it would otherwise have named, and a fetcher asking for it gets a real jpeg, a small picture with the image's address drawn on it, which goes through the image processor like any crawled picture. The seed page carries no image links.

At 0.1, one link in ten on each page is an image, so a crawl of a few hundred pages holds some tens of images and a search for media:image on its index returns them.