This isn't really a Mailmerge/fields question, but there are several things
you could try, e.g. if you have Office 2003 (maybe 2002) you could try
opening the TIFF in the Microsoft Office Document Imaging Application. In
Word, you can start with a blank document and use Insert|Picture|From File
as long as the TIFF converter is installed. You could try looking for free
software as TIF is a widely supported format.

However, the chances are that if you cannot read the TIF in an application
that is supposed to be able to read them, there is something wrong with the
TIF.

Peter Jamieson