I'd lost the original to an animated gif that I'd embedded in a previous talk's powerpoint slide. I tried clicking "Save image as..." but this gave me a lower resolution, scaled version without the animation. Seems there is a well known trick to finding original media in modern Microsoft office files. The .*x files are actually zipped directories. So unzip them to a folder using something like:
unzip myfile.pptx -d myfile/
Then you should find your media files somewhere in this directory. I found mine in: myfile/ppt/media/.