I do my thumbnails in bulk for my yearly update. I like to hand size & optimize the large images, then I just run them through photoshops automatics web gallery tool. All the thumbnails it generates are then automatically sized to fit with in a minimum height & width ratio.
Then I just wrap them each in a floated div. Ideally I'd have them centered both horizontally as well as vertically, but vertical alignment for DIVs in Internet Explorer like many CSS elements hasn't been implemented.
I could add, say black to the edges to make them all uniform like a letterbox film. But it just adds extra weight to the images, and one of these years I'd like to think Microsoft may implement the full CSS 2 spec, maybe.