5.2MB is indeed hefty, especially if he doesn't serve responsive versions. I'd tell him that if he uses 2x resolution images, he can get away with very high compression settings and still have the result look great. For sites with prominent visuals I've come to use five different sizes, from 5120px width for retina desktops (if possible) down to 1280x for mobile phones.
Hey, thanks for that. I'm not very knowledgeable about all this, but my first thought is that image size wouldn't really be a factor in this issue? My question (I think) is not about the quality of the image, but of the lack of adaptability in Shopify for the two platforms, desktop and mobile.