Use your FTP client to find this file on your host:
The simplest thing is to rename your own .jpg image to "kubrickheader.jpg" and over-write the existing one on the server. Note: the existing image is 760x200, so you'll have the best results if you make your own image the same size.
Otherwise, if you want to change the name of the image on the server, or use a different sized image, you'll need to find the appropriate .php file in your theme directory (probably "/wp-content/themes/default/header.php") and edit it with the new name and/or size. For instance, if you can upload "myimage.jpg" to /wp-content/themes/default/images, and then edit header.jpg so that it looks for myimage.jpg instead of kubrickheader.jpg.
Just play with the files until you get it right. If you backup any files before you change them, at worst you'll only need to restore the backup if things don't go as you'd like.