loop.php:
<?php if ( function_exists("has_post_thumbnail") && has_post_thumbnail() ) { the_post_thumbnail(array(480,175), array("class" => "post_thumbnail")); } ?>

function.php:
if ( function_exists( 'add_theme_support' ) ) {
	add_theme_support( 'post-thumbnails' );
	set_post_thumbnail_size( 480, 175, true );
	}