I actually wrote a macro to search the KM plist file for any embedded file paths and tag them so I could keep track of which files (mainly images and AppleScripts) that are currently in use in my macros. I've been using it quite extensively for a little over two weeks and it has helped me clean up my files. Check it out, it might help you keep track of which images are being used and which ones aren't.
Find Which Files Are Used in Your Macros and Tag Them Accordingly