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Years ago, a trick existed on Facebook. You could right-click the blurred image, select "Inspect Element" (or "View Page Source"), search for the image URL, and change the parameters from s160x160 (small) to v1600 (large). Facebook has since patched this. For locked profiles, the actual high-res image file is into your browser's memory. You cannot enlarge what isn't there. fb locked profile cover photo viewer exclusive
Which of these would you like? Or tell me a different lawful angle and I’ll create a detailed guide. Opt-in toggle Years ago, a trick existed on Facebook