Shutter is automatically activated on all links pointing to an image, with or without thumbnails, once the files are included inside the <head> section. No additional markup is required. This means that you can add Shutter Reloaded to your site at any time and all your previous images will work with the new viewer without the need of any modification.
Shutter Reloaded features resizing large images if the window is too small to display them with option to show the full size image, combining images in sets, redrawing the window after resizing, pre-loading of neighbor images for faster display and and “very good browser compatibility”. I can’t vouch for the compatibility thing at least on IE, because curiously, the site throws Error 403 if tried to open with IE, even version 7 and 8.