claim to "bypass" guards to show full-size images, but these are often just methods for viewing the public thumbnail in a larger format rather than breaking privacy settings. Privacy Settings and Visibility
The best rule of thumb for the internet applies here: Stay safe, respect others' privacy settings, and avoid third-party "hacking" tools.
When you upload a photo to Facebook and set it to "Private" (Friends only or Only Me), that image is stored on Facebook’s servers. When someone else tries to view it, their browser sends a request to Facebook’s server. The server checks two things:
The Truth About Private Facebook Profile Picture Viewers: Myths, Risks, and Reality