Powerpoint (Mac) transparency problem

I create a lot of scientific graphics in Illustrator which I then convert to Tif format. I then insert these tif files into Powerpoint and use the Select Transparency Color tool to drop out the unwanted background of the tif file. It works perfectly on my machine (Mac OSX 10.4.11) using Powerpoint 2008:mac (version 12.2.3).

However, when I send these files to clients (who also use Macs), sometimes the transparency instructions are lost when they open my files. Now they can see the ugly background of my inserted tif files.

Has anyone here encountered this problem? I realize it is probably a problem on my clients’ end but I don’t know what to tell them about how to fix it short of having them go from slide to slide reusing the Set Transparency Color tool. I checked and they are using the current version of Powerpoint:mac 2008.

Any suggestions would be most helpful.