The Quick Zoom Links are designed to make it easier for you to set the zoom quickly:
AutoFit - Will always resize new images to FIT them.
Select - Will zoom so that the current selection is maximally zoomed but still completely visible.
Stretch - WIll zoom so that current image is maximally zoomed but still completely visible.
Fit - sets the zoom to maximum value that will make the image completely visible but will not go over 100% zoom.
The Zoom slider lets you quickly set a specific zoom factor:
