arthurdent, do you have firebug for firefox? If not, install it and check the console to see if there are any javascript errors (Chrome also has a build in console that will report any js errors). If there are any errors let us know what they are.
Another thing to remember, lightbox.js probably requires jQuery. It is important that you also load jQuery and make sure that it is loaded before lightbox.js. I usually include my javascript files in my controllers init function like so
class Page_Controller extends ContentController {
public function init(){
Requirements::javascript(THIRDPARTY_DIR.'/jquery/jquery.min.js'); //include jquery bundled with ss
Requirements::javascript('themes/'. SSViewer::current_theme() .'/javascript/lightbox.js'); //link to js file in current theme folder
}