try to limit avatars to 50K and signatures to 100k and make everyone post thumbnail instead of full images.

and host them externally as everybody told you here earlier.

Put your site name on every image you host for your forum to prevent leeching it
or apply this
http://www.besthostingforums.com/for...ead.php?t=1732

Check out if someone DDos your site or not.

All of these could abuse your bandwidth.

If everything fine, then your host play with you and want you pay extra bandwidth fees.