You're right, the PagePicture method is only for web pages.
We don't have script examples of how to capture other types of scrollable windows, as it's a non-trivial task. However, there are http://www.google.com/search?q=scrolling+window+capture that can do this, for example, http://www.ducklink.com/p/free-screen-capture-tool/ (free). So, I recommend using these utilities in your tests.