You might find a utility to convert something else like .WMF or .BMP to .TIF if you buy a cheapie clip-art disk, no guarantees though.
Otherwise, this might be an INSTALL issue. I don't recall whether .TIF is supported for everything, but when you install Office, there is a point in a custom install where you can specify a pot-load of graphics interpreters, one of which might be .tif - and if so, you might be able to even get WORD to store a picture in a different format than the one in which it was imported. If you do a DEFAULT install, not all graphics interpreters are loaded and therefore you might not have .tif as an option.
Granted, using a Word-based import and subsequent export would be uglier than sin and not nearly so much fun. But it might get the job done. Better, though, is that if Word CAN do it, you can automate it with VBA.
Just don't hold your breath waiting for it to work, that's all.