20% of the "max" per visitor for pics viewed sounds about right, but it depends on the pictures as well. A hot-ass girl is going to usually get more people looking at the images than an ugly one.
so figure your page size for the HTML and all the thumbs, and multiply that by the # of visitors you plan to get. That's concrete.
Then figure your total for all the full size images, and take 20% of that and multiply by the # of visitors.
add the two together, and you should be within prolly 10-20% for your bandwidth usage pretty reliably.
-Phil