What is the URL schema of Tumblr images?

What is the schema of a image-file at Tumblr? (I don't mean HTTP) I've only figured out that the domain of the servers where images are stored is <n>.media.tumblr.com, where n is a number between 1 and 31 and the name of the image file is prefixed with "tumblr_.

I'm asking because I want to find URLs that refer to the same image.

EDIT: I'm also processing URLs from other sources, not only Tumblr.