There are two full screen buttons. Left button activates full screen preview of the input (original) image, right button is for output image preview.
Keyboard commands available in Fullscreen mode:
⌘+ keys for zoom in image.
⌘– keys for zoom out image.
⌘0 for actual size view.
⌘9 for fit to screen view.
⌘1 to toggle input and output images.
You may use PageUp and PageDown for image browsing.
Use ⌘F or Esc keys to exit from Fullscreen mode.