Display issues (content “squashes up” when browser window is resized)

On my site, the content “squashes up” when you resize the browser’s window. Similar problems occur in some browsers (particularly on Macs) which have built-in toolbars or menu bars on the sides, etc.
For example, the container div shrinks but the header image stays put.
I hope that I’m making sense.
Can someone help with this?

The URI is http://www.vaguely.org/blog
CSS at http://www.vaguely.org/blog/weblog.css