Is there an option in the ACP like in phpbb3 ?

Enable GZip compression:
Generated content will be compressed prior to sending it to the user. This can reduce network traffic but will also increase CPU usage on both server and client side.

Or you can enable GZip via htaccess
http://www.jhuskisson.com/articles/a...e-via-htaccess

You can disable image hotlinking, that may help a little
http://www.besthostratings.com/artic...t-linking.html

Also your background image forumbg.jpg is 60.1KB, that could be hosted on tinypic.

Check in awstats in cpanel, down the bottom it will show you what files/images use the most bandwidth so you have a better idea what to optimise.