Most "private viewer" tools fall into these categories, ranging from ineffective to malicious:

If the photo is set to "Friends Only" and you are not friends with the target, the server simply refuses to send the image data. There is no "back door." You cannot trick the server into sending an image to a client that hasn’t presented the correct credentials.

Downloadable "viewer" software or browser extensions often contain viruses, keyloggers, or spyware that infect your device. Survey Scams:

The search for a "Facebook private photo viewer" often starts with a simple curiosity: seeing a picture that has been hidden behind a digital wall. However, the reality of these online tools is far more complex—and often more dangerous—than their marketing suggests. The Myth of the "Magic" Link

Suddenly, his laptop fan roared to life, a jet engine taking off on his desk. The screen began to flicker. Files on his desktop started opening and closing rapidly—his tax returns, his thesis draft, photos of his family. It was like a ghost was rifling through his filing cabinet.

He clicked the first link. It looked like it hadn't been updated since 2012. Neon green text on a black background. A single input field asked for the target’s profile URL.

Engaging with websites that promise to show you private photos comes with significant risks to your digital safety:

If you genuinely need to see photos on a private account, there is only one legal, ethical method.