Hey,
we use the prettyphoto lightbox: http://www.no-margin-for-errors.com/projects/prettyphoto-jquery-lightbox-clone/ - I'm not very familiar with the lightbox code and I can't provide a solution but maybe you can get help here: http://forums.no-margin-for-errors.com/
If you'd like to deactivate the gallery thumbnails open up js/avia.js and replace
elements.prettyPhoto({ "theme": 'premium_photo', 'slideshow': 5000 });
with
elements.prettyPhoto({ "theme": 'premium_photo', 'overlay_gallery': false, 'slideshow': 5000 });
Edit you can try to replace following line in:
if(pp_images.length>30)
with:
if(pp_images.length>30||pp_images.length<2)
I haven't tested the code but I hope it works.