Unable to View Signature Logo in Outlook 2007

I don’t have an idea what is causing the issue in our emails. We are using Exchange 2003 server with Outlook 2003 and Outlook 2007 that is already configured. The issue is if we receive an email from a company that sends logo as their signature, the receiver gets an Outlook authentication box if the said email will be opened. In Outlook 2007, the forwarded attachment won’t show and has a red x to where the logo appears. Can you please help me figure out what is the probable cause of this problem?
