you'll find it on the option to edit the template

then 

Hello Marianne,

Further to what Si has suggested you need to right-click on the filename and select 'Remove' from the pop-up menu which appears.  This applies whether the image has been added on the 'Header' tab via 'Edit Graphic Template' in General Settings, or by using the 'Edit' properties button that Si referred to.

Kind regards,

Paul