Why wouldn't you show pictures directly from the Finder?
Seems like you haven't discovered Quick Look - it's great for images!
Quick Look (tap your space bar) with a single file, or select multiple images (or whatever), and QuickLook instantly to a slide show. Click the full screen icon if you like. Let the slide show run automatically, or click the pause button to go from image to image manually.
And - it all works directly from the Finder, as you were asking...!