[Theme: Twenty Eleven] Best way to reduce the header image height?

This is an amazing theme. One of the best visual designs I’ve seen, plus it’s extremely efficient.

But I’d like to reduce the height allocated to the header image. On my computer (a 13″ Macbook), it pushes all the content off the bottom of the screen. I suspect that’s the same for other visitors.

And so, without destroying the theme too much, I’m wondering what the best way would be to reduce the image’s height.