Let the user have separate control of desktop view and mobile view for image sizing. Give them the option to say "width" and let them choose the units (%, em, px, etc.)... Let it be the responsibility of the user to make it look good so they have the flexibility to not have it look awful by default. Resizing the image down per your guidelines on retina based devices (almost everything nowadays) makes the desktop display pixelated so it can look not terrible on mobile. At the very least, set it so retina based clients on mobile get the images at 50% the image size so we can minimize the damage of pixelation.
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