Featured image problems

Hello.

I am having a problem with a child theme I am trying to build using twenty ten. I have changed the size of the header image and that works fine with all images but I cant seem to use a feature image. I have created a functions.php file with containing;

<?php

	define( 'HEADER_IMAGE_WIDTH', apply_filters( 'twentyten_header_image_width', 980 ) );
	define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'twentyten_header_image_height', 224 ) );

?>

This works brilliantly to change the size but all my feature images become the the same as the header image. Anyone know where I’m going wrong?

Many thanks