These are not "free viewers" but paid parental control or monitoring apps. They require physical installation on the target device and do not "hack" Facebook servers; instead, they capture what is displayed on the device's screen. Warning: Scams and Security Risks

There are two main categories of tools you will encounter online, each with significant limitations:

Historical loopholes that allowed viewing full-size photos through direct URL manipulation or "graph" searches have largely been patched.