The images are all stored in UI\Default\images. You'll need the DDS plugin for Photoshop/Paintshop Pro available from nVidia's site. (It's in one of the download categories, I forget which offhand...free downloads, or utilities, or something like that)
As far as figuring out which window uses which files to grab its images from, that's defined in the XMLs in UI\Default, but without knowing more about what exactly you want to edit that's about as specific as I can get.
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