Wide header image in Twenty Ten

Hi there.
I need help to adjust the width of the header image on my website. I use the theme Twenty Ten, that suits my needs well. I am not used to CSS and PHP coding. I have made a child theme and a functions.php with this code:

<?php
/**
* ABC org child theme functions and definitions.
*/
add_filter(‘twentyten_header_image_height’,’my_header_height’);
add_filter(‘twentyten_header_image_width’,’my_header_width’);
function abc_header_height($size){
return 250;
}
function abc_header_width($size){
return 1900;
}

The purpose of this is to allow my header image to size 1900 px wide and 250 high. 250 high seems to be ok. My problem is that the header image starts at the same place as before. I want it to be center aligned. Wider than the menubar and equal addition on both sides of the menubar. The content with text on the site will still be 940 wide.
Here is my website: https://www.abcorg.no
Anyone know how to fix this?