The use of Frontpage or Dreamweaver are rather frowned upon here as they can screw up phpBB2 files quite badly so be sure to make backups.
Open the template file in question and copy the code. Paste it between the body tags in Frontpage/Dreamweaver with the programme in code mode. Now go to the Design mod so that you see the page. Do not let Frontpage connect to the internet or it might well screw up some image paths by replacing them with cached locations. Make the changes that you want in the design view (I use the split view in FP so that I can see the design and the code together) and then copy the code that is between the body tags back to and overwrite the original template code.
ftp to the server and test - If screwed up then quickly replace the original.
The truth is out there.
Unfortunately they will not let you anywhere near it!