This is an increasingly worrisome issue. I installed ICQ a few weeks ago, and before I had a chance to configure it I received several offers to visit pornographic sites. I know of no product that will allow you to block image transfers, and my search revealed nothing. Require confirmation for people to add you to their buddy list. This significantly cuts down on any such offers.
Technical solutions are no replacement for good parenting practices, however.
Children's use of the computer ought probably to be restricted to times when a parent can supervise. As children get older and more mature, they can be taught to handle these situations. Technical solutions are notoriously ineffective. Even if images were blocked, someone could still send a URL for an image. NFS A debate is raging over a firewall within our organization and making up the internal part of a DMZ zone.