var i = 0;
var lightbox = document.getElementsByTagName('a');
for (i=0; i<lightbox.length; i++){
 if (lightbox[i].className == 'galThumbnail'){
  var newhref = lightbox[i].href.replace(/(.*)img\.php\?(.*)/, 'show_img.php?$2');
  lightbox[i].href = newhref;
 }
}