How do i format my custom header so it works correctly in carrington?

I am using the carrington theme to set up my blog and portfolio website at slangdesign.com

I want to modify the header with a different image but every image I have attempted to use messes up the theme.

The default image is 960 x 200 pixels but only 95 pixels of it shows on the screen. When I use an image that is 960 x 200 pixels it shows the entire image on the screen and displaces everything, making it look horrible.

[url=https://img200.imageshack.us/my.php?image=testimage2.jpg][img]https://img200.imageshack.us/img200/2254/testimage2.th.jpg[/img][/url]
[url=https://img132.imageshack.us/my.php?image=testimage.jpg][img]https://img132.imageshack.us/img132/4861/testimage.th.jpg[/img][/url]