Adding a new rendition to a content item in Stellent

We're running Stellent Content Server 7.5. We're using it primarily to store images and create various renditions of those images.

I've managed to figure out how to use the web services to upload a new file to Stellent. I'd like to be able to (programmatically - not through the UI) add a new rendition to a new or already-existing image set.

Basically, I want to mimic the "Add a New Rendition" action available through the UI. (Through the UI, Browse Content->Contribution Folders and find the image, click Rendition Information. On the Rendition Information tab, in the actions drop-down, select Add a New Rendition. This allows you to add a new rendition to the set of renditions for the particular image.) I want to do this, but programmatically - through the webservices.