Powerpoint Image Extractor V1.2 -
PowerPointImageExtractor (free) - Download - will extract all images (and .wav files) from a presentation and save them in a separate folder (as .png files).
The user interface is completely nonstandard (typical Frenchman!) but the functions are comprehensible and it works very well (just tried it).
Mac
You could then zip the images if you wanted and get 'em all in in a single file (.png is already compressed, so you won't be much smaller than the sum though).