I assume you downloaded the distro files for this theme to your hard drive? Look in that theme’s /images folder and find the header graphic. Make a note of its dimensions. You don’t want yours wider or narrower than the original but it can *probably* be taller or shorter (top to bottom) than the original if need be, but shoot for identical dimensions, it will make your life easier. You may need to crop your intended image to achieve this. Then save your image over the original if you don’t plan to ever use it. Then just upload the new image to your web site in the same place as the original. If you do that, you’re done.
However, if you want to give your image another name, you’ll need to also go into your stylesheet (style.css) and find where it calls the original image. Substitute your image name for the original and make sure if height and width are declared that you put your image’s dimensions in. Don’t forget to upload the new image to the /images folder of your theme and make sure that you update that stylesheet. (You can edit that directly from within the WP Dashboard if you want.)
