The Preview slideshow has a very simple floating control. Once Preview opens with all of the selected images (left pane in Preview screenshot below), launch the slideshow by selecting View | Slideshow Launch Slideshow from Menu in Preview Launch Preview for Multiple Files Selected in Finder The Apple Preview utility program has a slideshow capability and can be launched from Finder by selecting images in Finder and right clicking to bring up the menu - select open to open all the selected files. Opening and Playing selected image files with Preview in Finder The Option Spacebar method of displaying a slideshow has the advantage that it is simple, quick, and easily accessible from Finder, but gives minimal control over the slideshow. On the floating slideshow control, the left pointing arrow goes to the previous slide, the two vertical bars icon pauses playback (solid right pointing arrow resumes playback), the right pointing arrow advances to the next slide and the four squares icon brings up the “index sheet” view of the slide show which shows thumbnails for each slide on a single page: Index Sheet in Option Spacebar Slideshow Option and Spacebar Keys on Mac Keyboard Floating Slideshow Control for Option Spacebar This will play the selected images as a slideshow in full screen mode. Then, simply press the Option and Spacebar keys on the keyboard. Using Option Spacebar to play selected images in the Finder Select Image Files in Finder
These ways of creating and displaying slideshows were tested in detail on a MacBook Air running Mac OS X version 10.12.6 (macOS Sierra), Apple Photos 2.0 (3161.4.140), Preview Version 9.0 (909.18), iMovie version 10.1.7, and System Preferences Version 14.0 (the Desktop & Screen Saver control is part of System Preferences).
This article also discusses how to avoid interruption of the slideshow by either the Mac Screen Saver or Energy Saver/Sleep when using an external display and security and privacy issues for slideshows.