I stumbled on this today and found for me the problem was using a PNG image.
When I used a PNG, the image looks fuzzy and low quality. When I converted it to JPG (losing the transparency), it looks pretty much the same as it did in ppt.
I tested it with a PNG that didn't have any transparency and had the same outcome. Moving the compression to 'low' didn't have any noticeable impact on the quality of the PNG.
