Planetary Society blogger Emily Lakdawalla has recently initiated a series of classes on how to process space images. The classes are being presented live as WebEx conferences and can be downloaded and reviewed as recordings any time. Online, web-based tutorials are also available.
OPUS and other Rings Node resources are featured in the most recent class:
III. First Voyage into the PDS (March 30, 2010)
It's a really great informative introduction to PDS data and imaging. The OPUS tool demo is at 49:00 minutes. Easiest way to view is to click the Stream link.