You would have to enable HTML in sigs and bypass the filter that provides the security that you need to prevent malicious code. Disabling that filter is a serious risk and one that shouldn't be taken lightly.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.