Looking at your code on your page I do not see where you are calling the lightbox.js script - that needs to be called right before the body close tag.
Also lightbox help states : If you already use jQuery on your page, make sure it is loaded before lightbox.js. - ◦Include the Javascript at the bottom of your page before the closing /body tag:
I used the lightbox-plus-jquery.js and called that just before the close of the body tag and it worked. I called the lightbox-plus-jquery.js just before the close of the body tag because it has both jquery and lightbox combined.
I am still working on the page and I am just using html not .net but the lightbox would work the same. If you look at my source you will see the script call right before /body tag.
http://just-in.com/recycledDresser/index.htm
Hope this helps.
Debra W.