PowerPoint Gurus: can you set opacity for images or videos?

[OpalCat]

I know about the "washout" setting but that seems to washout the picture to white, whereas I want to be able to see my background image through it. What I am trying to do is have an animated watermark, either via animated gif or a video clip set to a low opacity/transparency. Can this be done? The clip is about 80 frames long so I would rather not fake it by editing each frame of the video...