wordpress - get the first image on the_content

function get_the_first_image() {
	$files = get_children('post_parent='.get_the_ID().'&post_type=attachment&post_mime_type=image');
	  if($files) :
	    $keys = array_reverse(array_keys($files));
	    $j=0;
	    $num = $keys[$j];
	    $image=wp_get_attachment_image($num, 'large', false);
	    $imagepieces = explode('"', $image);
	    $imagepath = $imagepieces[1];
	    $thumb=wp_get_attachment_thumb_url($num);
	    print "<img src='$thumb' class='thumbnail' />";
	  endif;
	}