Are you using the default WordPress Twenty Ten theme?
The Blogarista has a great tutorial telling you how to change the height and width of the header so you can use your own image. She said,
Since a lot of readers use the basic pre installed WordPress Twenty Ten theme I thought I would do a series of tutorials on how to customize the theme. Today, I’m going to cover the header width and height. It’s easy to change and will allow you to use your own header size instead of the generic header provided.