add_filter( 'attachment_link', 'direct_image_urls_for_galleries', 10, 2 );

function direct_image_urls_for_galleries( $link, $id ) {
	if ( is_admin() ) return $link;

	$mimetypes = array( 'image/jpeg', 'image/png', 'image/gif' );

	$post = get_post( $id );

	if ( in_array( $post->post_mime_type, $mimetypes ) )
		return wp_get_attachment_url( $id );
	else
		return $link;
}