Header alignment set to "top center," but image still cuts off from the right

I've gone around and around trying to solve this issue. I know my fixed-width header is going to be cut off on some browsers. I thought the "Header Image: Alignment" control would let me choose whether it gets cut off from the right, the left, or equally from both sides. The latter is what I'm trying to achieve, but I can't seem to figure out how to do this. Currently, the alignment is set to "top center," the the image still cuts off from the right. Is there a way to ensure that the center of my header image remains in the center of the page regardless of browser size, and that any excess is cut off from both sides equally, rather than all from one side?

I'm using Atahualpa 3.5.1 and WordPress 3.0.

Thanks!