Functions.php in child theme

Hi

I would like my header image to be 940×300 so I tried to create this in the fucntions.php in my child theme.

<?php
/* customs header image sice */
function twentyten_setup() {
define( ‘HEADER_IMAGE_WIDTH’, apply_filters( ‘twentyten_header_image_width’, 940 ) );
define( ‘HEADER_IMAGE_HEIGHT’, apply_filters( ‘twentyten_header_image_height’, 300 ) );
}
?>

It removes the header image all together along with the backend option to add a header image.

Would anybody be so kind as to point me in the right direction on how to override a function the right way?

Thanks a lot
Claus