Choose to Display Post Thumbnail?

This is modified code from twentyten that does what you’re looking for. Put this in header or wherever you want the image to run. See string post-thumbnail — that is size. Make sure it is defined as 940 x 180 in functions.php

if ( is_singular() && current_theme_supports( 'post-thumbnails' ) &&
        has_post_thumbnail( $post->ID ) &&
        ($image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) &&
        $image[1] >= 940 ) :
    // Houston, we have a new header image!
    echo get_the_post_thumbnail( $post->ID );
elseif ( get_header_image() ) : ?>
    <img src="https://wordpress.stackexchange.com/questions/101849/<?php header_image(); ?>" width="940" height="180" alt="" />
<?php endif; ?>

It falls back to header_image if that is set. Feel free to change sizes.