How can I change my header on my theme

http://codex.wordpress.org/Designing_Headers

it’s going to vary by theme. That link gives some general advice. A link to your site will get you more specific help….

looks like it’s going to be in your header.php file…at least that’s where the image is called from.

You could just make an image called header.jpg and upload it to your theme’s images folder (deleteing the old header.jpg)

that’s the simplest explanation

right, it will need to be the same size as the old one (for simplicity sake…this can be altered with justa bit of tweaking)