Lokesh Dhakar has put together a marvelous little hack called Lightbox to display large-format (“click to enlarge”) images on a web page. The graphic loads up over the existing page in an effect similar to OS X’s Dashboard.
Places images above your current page, not within. This frees you from the constraints of the layout, particularly column widths.
Keeps users on the same page. Clicking to view an image and then having to click the back button to return to your site is bad for continuity (and no fun!).
Be sure to click through and check out his demo. Just brutally, terminally cool.