Ah, okay. Thanks. I figured Doku was doing some sort of indexing or something.
It does. It records image data, keeps a history of changes including storage of old versions. Much the same as it does for pages. You can see this if you open the top-level media manager. If you click on an image, at the right there will be tabs for image data and history.
It's useful if you ever want to recover a previous version of an image.