lightbox issues

i have uploaded some images to my products. some are very large and when viewed on the site as a lightbox, they can expand beyond the browser. but when i am editing the actual product page, i have the ability to view the same image in a lightbox. however, this picture does not expand beyone the browser.

how can i get the front-end lightbox to fucntion like the back-end lightbox?

thanks in advance!