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It is extremely easy to batch import images into Keynote. Set the View in the presentation to Navigator (using the View menu), which will show you the slide thumbnails in the left sidebar. Now select your image files in the Finder, and simply drag them into the thumbnail sidebar. A new slide will be created for each image. You can ensure that large images will be scaled to fit on your slides by going to the Keynote Preferences, in General, and check the option "Reduced placed images to fit on slides".