WordPress 3 and the Twenty Ten theme are very easy to modify through the Appearance section of the dashboard. Buy clicking Header in the Appearance section you can very easily choose a different image or upload one of your own.

The image size is 940 x 198 pixels and there is no obvious way of changing these dimensions.

I can live with the width setting but I find a graphic header banner of ~200px too big and takes up too much screen space. I often use 100px for a banner and for this website I wanted to use 130px, so how do you change the height?

A quick search on Google lead me to this link –

http://www.bloggerstemplate.com/2010/06/20/how-could-resize-the-header-of-twenty-ten-theme

Which tells you what to do, which is –

1. Click on Editor in the Appearance section (left)
2. Click on Theme Functions in the Templates section (right)
3. Scroll down until you see the height and width definitions and change 198 to the value you want