I didn't look at the gallery pages the first time. I would either get rid of the irregular buttons banners or link them. It's fairly simple to use one link to coverall the text and buttons you have at the top of each gallery.
Remember you want to sell your sponsor at the top of your index page...if they get inside you are doing something wrong.
That said you can't do anything about freeloaders that burn your BW. My logs tell me how much time the average surfer spends on my pages and the last one is 1 hour+ and that is always a fraction of 1%.

The bulk of my surfers spend about 2 mins.