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Keynote presentation documents are actually "packages" of files that the Finder presents as a single file. The slide backgrounds should be accessible in the presentation package. To show the contents of the package, in the Finder Control-click on the file, and choose "Show Package Contents". In the Finder window that opens you should see a variety of files, and some of those files should be the background images used in the presentation. I would suggest copying (not moving) those images to another location, make the necessary corrections, and the set them as the slide backgrounds from within the Keynote application. (The alternative is to make the changes directly to the images in the package, but that may risk corrupting the document.)