In order to access media files embedded within pptx files, follow these steps (and they work on both Windows machines and Macs):
Make a copy of the pptx file.
Rename the pptx file – change the extension from ‘pptx’ to ‘zip’.
You will probably get a message warning you about the change of extension. Make sure you allow the change so that the file name will end in ‘.zip’.
Extract the zip file (or browse the zip file contents). You will see a directory structure now present in the folder. Navigate to ppt > media and all your image and media files will be present. You can now copy these files (and convert, if necessary) for use in other presentation mediums.