It is recognised that forum admins and site owners face a particular problem in policing this aspect of the copyright law. The High Court recently ruled that Article 14 of the EU eCommerce directive provides some protection from litigation where site admins have no knowledge that the material posted infringes copyright (
http://www.out-law.com/page-10902.
This is somewhat similar to the protection afforded in the USA by the Digital Millenium Copyright Act (DMCA), but in both instances, it is vital that you as a site admin act immediately you receive notification that one of your forums contains copyright material which has not been licensed. It then becomes a matter for the copyright owner whether they wish to take action directly against the alleged infringer. To do this they might request that you reveal any registration details you may have concerning the poster, depending on whether you operate a members only system. You have to be careful here not to break your obligations to the poster under the Data Protection Act, by revealing information about them to a third party. Generally I would recommend that you decline to hand over information to a copyright owner unless they get a court order (known as a Norwich Pharmacal) requiring you to do so. If it is Getty, then they have the resources to obtain court orders, so they shouldn't need telling about this.
If you do get threatened with action by a copyright owner in the first instance, you can point out that legally speaking they will need to prove that you knew that the item a. existed on your forum, and b. that it infringed copyright, before they can have a reasonable chance of a case against you. If they still want to pursue the matter, speak to a lawyer who specilies in IP law.
If you are not already familiar with them, you should read the
Electronic Commerce (EC Directive) Regulations 2002 for more details of your obligations as a provider of information services.