For a header to consist of an image and be rounded at the same time (atleast in good browsers), using border-radius to crop a box and having the image as a background-image is pretty much the best (or only) option now.

And if you want to have the box clickable without going for JavaScript, you have to add display:block to the link and size it properly. As long as you don’t have too rounded corners, this works just fine with very little code; the only problem is that it seems impossible to get the link to be anything but squared with just CSS.