Just like Word, it resamples (using a rubbish algorithm) any images to the dimensions allowed by the template/slide layout, assuming the DPI value saved in the file. If you supply images at exactly the right DPI and pixel size for the output device, and the person who puts them in knows what (s)he is doing and sizes them at 100%, then they will look fine. Otherwise, they will look soft and fuzzy.
Perhaps the image was dropped into a 'container' smaller than the pixels would dictate?