Some WP themes are easier to change than others. Yours has a gradient background that from a first look seem to make changing the image difficult. If you give me the name or link to the theme it will be easier to help you.
I have made changes in WP theme headers for my blogs. Check my headers at
fightclub.gaijindo.com and
foodfilms.org you will see it is the same theme only top image changed. This was easy, the original image was a jpg named header.jpg found in the folder of the theme. So I just made my own image in the same size, named it header.jpg and replaced it.