Known as a "no-login" private viewer, PeekViewer uses session mirroring to retrieve data without alerting the user.
If you don't want to install an extension, several web-based tools allow you to simply paste a profile URL to retrieve the image.
You don’t need a secret URL to view a friend’s profile picture history—Facebook has a built-in feature.
Facebook allows you to see the profile picture of anyone who shares a mutual friend with you, if the picture is set to "Friends of Friends." However, if it is set to "Friends Only," no URL, script, or code will bypass it. It is encrypted server-side.