Well, I think that sounds easy enough to prove or disprove. He could just send you an image file, both pre-compressed and with his magic touch. However, if he refused for whatever reason, that’s when I’d start to ask questions.

I recently learned about image compression and its limits when I tried to reduce file size by packing PNGs into a 7z-archive. You may be laughing now but I really didn’t know PNGs are already compressed perfectly and there’s no way to squeeze them more without loss.

I spent about half an hour using the highest lossless compression rates only to notice that the archive was about a few kilobytes smaller than the raw file. That’s time I won’t get back. But it serves as a good lesson.