Override a parameter from functions.php in a child theme

I looked all over Google, but I can’t find the solution.
I’m using Whiteboard as a framework, and I’ve created a child theme. In the main theme’s functions.php I have the following lines:

// custom background support
	add_custom_background();

	// custom header image support
	define('NO_HEADER_TEXT', true );
	define('HEADER_IMAGE', '%s/images/default-header.png'); // %s is the template dir uri
	define('HEADER_IMAGE_WIDTH', 1068); // use width and height appropriate for your theme
	define('HEADER_IMAGE_HEIGHT', 300);
	// gets included in the admin header
	function admin_header_style() {
	    ?><style type="text/css">
	        #headimg {
	            width: <?php echo HEADER_IMAGE_WIDTH; ?>px;
	            height: <?php echo HEADER_IMAGE_HEIGHT; ?>px;
	        }
	    </style><?php
	}
	add_custom_image_header( '', 'admin_header_style' );

I would like to change
define(‘HEADER_IMAGE_WIDTH’, 1068); and define(‘HEADER_IMAGE_HEIGHT’, 300);

I couldn’t find the solution in http://codex.wordpress.org/Child_Themes since the option to override is

if ( ! function_exists( 'theme_special_nav' ) ) {
    function theme_special_nav() {
        //  Do something.
    }
}

But in my case is not a function.
Any help would be much appreciated.

Thank you