Maybe put dithering images outside the paywall and full pictures within and in your ad block placeholder a message that says pro users get the real images.
You sell it as quality. Think of ordering the quick paperback printing with black and white photos vs getting the full quality hardbound with a glossy photo folio in the middle. Premiumification vs free to read. It is also egalitarian as users aren't paywalled, get access to the content and only pay if they want to use the extra bandwidth.
