how to change header image size on twenty ten

Hello,
I know this question has been already asked but I seem not to find a suitable solution for my website http://www.paperheartmusic.net

I changed the size here, in the functions.php :

// The height and width of your custom header. You can hook into the theme’s own filters to change these values.

// Add a filter to twentyten_header_image_width and twentyten_header_image_height to change these values.

define( ‘HEADER_IMAGE_WIDTH’, apply_filters( ‘twentyten_header_image_width’, 940) );

define( ‘HEADER_IMAGE_HEIGHT’, apply_filters( ‘twentyten_header_image_height’, 300) );

// We’ll be using post thumbnails for custom header images on posts and pages.

// We want them to be 940 pixels wide by 300 pixels tall.

// Larger images will be auto-cropped to fit, smaller ones will be ignored. See header.php.

set_post_thumbnail_size( HEADER_IMAGE_WIDTH, HEADER_IMAGE_HEIGHT, true );

Then uploaded an image of that size and chose it as header image.
the problem is that the image is all squished and looks awfull as the correct size was not chosen.

It always remains at 100 px height and not the 300 that I would like to have.

What else should be done ?
Should I also edit the header.php ?
Thanks for your help already…
I’ve been on this for hours, trying and retrying and nothing ever works. I even got like a 300px empty rectangle between the menu and the content of the page. I finally got rid of that one but still I can’t simply put an image of 940/300 as a header…