http://osxdaily.com/2012/09/06/8-tricks-mac-os-x-full-screen-slideshow/Select a picture or group of images from the desktop, then use the following:
Option+Spacebar to launch image(s) into the full-screen slideshow mode
Spacebar to pause/play the image slideshow
Left Arrow to go back, Right Arrow to go forward
Gesture with two-fingers left to go forward, two-finger gesture right to go back
Option to view smaller images at actual size
Click “Index Sheet” to view thumbnails of all images in the slideshow
Click “Add to iPhoto” to import the image into iPhoto
Hold Control key and use a two-fingered back or forward swipe in to zoom into the photo
Escape to exit