ok, how much html/css knowledge do you have? I can walk you through a couple of things that could lend a fix... basically the page thinks the gallery is wider than it is, so it's bumping your sidebar down. You'll need to add some css and possibly some html to fix the problem.
I'd start by adjusting the width property of the gallery div, using
.gallery_box_single {width:605px;}
You'll need to add this to your css in style.css