I have tried doing what you are doing, but sometimes it is successful and sometimes it is not. I don't have a definite answer to your question.
But try opening your graphic in PSP and go to
>> COLORS
>> Set Transparency Color
>> Set Transparency Color to current background color
And save it as a .bmp. I don't think I have ever been able to get the .wmf to work with transparency.
Hope this helps.
Oliver Scheurer
Aegis Solutions, Inc.
Phone: (919) 861-0103 Ext. 231
oscheurer@aegis-solutions.com