Get image from post’s gallery

You can get the attached media to a post using get_children. IE: get the first attached image for post ID == 14

$args = array( 'post_mime_type' => 'image',
               'numberposts'    => 1,
               'post_parent'    => 14,
               'post_type'      => 'attachment' );

$first_attached_image = get_children( $args );