Center the Banner Images

Folks,

I find it really hard to come up with pictures that work well when they are as long and thin as the banner size. I would really like to have my pictures retain their aspect ratio, expand to the correct height and then float to the centre of the banner page. I am a bit of a novice in "css" so I can't figure out how I might do this. Any thoughts before I dig out my image editing tools and start creating wide images with lots of white space at the side...

Dave