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 –
Which tells you what to do, which is –
- Click on Editor in the Appearance section (left)
- Click on Theme Functions in the Templates section (right)
- Scroll down until you see the height and width definitions and change 198 to the value you want