How can I see theme variation images in Keynote?
The built-in files for Keynote are stored within the package. Go to:


  /Applications/iWork '09/Keynote.app/Contents/Resources


EDIT: The theme files themselves, at least on my machine running iWork '09, boxed version, are stored here:

/Library/Application Support/iWork '09/Frameworks/SFRendering.framework/Versions/A/Resources/Frames


Each theme contains a file with a suffix of sfrframe. Inside that file are the images for each theme.

Also under /Resources is a folder called /Themes. Within it are several theme files, shared images, etc. Each theme has several .kth files, which contain theme files and metadata.