Let’s start by changing the standard TwentyTen header. The default size is 940 x 198 Pixels, so the easiest way to replace it is to create a new header in the graphics program of your choice to fit exactly that size.
Once you have created the graphic, changing it in WordPress is easy. Just follow these steps (click on the images to enlarge):
1. Log into the WordPress Dashboard with the administrator account.
2. Select “Header” under “Appearance” in the dashboard side bar.
3. Click on “Browse” (shown here in my German OS as “Durchsuchen”).
4. Select the file that you created for the header and click on “Upload”.
5. The new header will be previewed.
6. Return to the site and the header is now active!