Your lightbox is in slideshow mode with autostart true, change to false will solve the problem.
$(".gallery:first a[rel^='prettyPhoto']").prettyPhoto({animation_speed:'normal',theme:'light_square',slideshow:3000, autoplay_slideshow: false});
UPDATE
If you do not want to use slideshow can simply change the line in question to:
$(".gallery:first a[rel^='prettyPhoto']").prettyPhoto({animation_speed:'normal',theme:'light_square'});