How to disable thumbnail sizing?

I’m using a theme which takes a featured-image attachment and plugs it into a page. The images I use are already correctly sized for the space, so I really don’t need the images to be resized, and in fact I suspect this code is really slowing my page load down. How do I simply have the featured image used “as is” with no resizing function? See code below (“featured_540x270” is defined in functions.php for this theme):

‘$post_img_big = wp_get_attachment_image(get_post_thumbnail_id($post -> ID),’featured_540x270’,” );
$img_description = de_getPostImageDescription($post -> ID,get_post_thumbnail_id($post -> ID));
}
else
{
$post_img_big = get_first_image($post -> ID,’540×270′); ‘