dont look for feature image but find a Custom field?

i have this code in functions.

this theme is looking for feature images in order to display thumbnails like this

/* Check if this video has a feature image */
	if(has_post_thumbnail() && $thumb = wp_get_attachment_image_src(get_post_thumbnail_id($post_id), $size))
		$thumb_url = $thumb[0];

}

i had always used custom field to my images example is my_thumbs wew.org/heart/HEART_ASDcombo2.jpg