Show image exactly defined to a width

If $content_width is the problem, you could set it to something different if you’re viewing a single HQ post.

function wpse99587_single_hq_post() {
    global $post;
    if( is_single() && in_category( 'HQ', $post->ID ) ){
        global $content_width;
        $content_width = 900;
    }
}
add_action( 'wp', 'wpse99587_single_hq_post' );
    // I dithered on where to hook this function;
    // if there's a better hook to use, please correct me

This assumes that the $content_width variable is global.

See the Codex pages for is_single() & in_category() for more information.