Facebook’s Transparency Report and Terms of Service (Section 3.2) explicitly forbid:

The desire to see a hidden photo is natural, but the risks—identity theft, account loss, malware, and legal action—far outweigh any possible reward. Respect the privacy that Facebook’s lock feature was created to protect.

If you are a journalist or researcher trying to verify someone’s identity:

The desire to see a locked profile picture usually stems from curiosity—checking out an old friend, a potential date, or someone who has blocked or restricted you. Scammers exploit this curiosity by:

The short answer is no. The long answer is that while you cannot "hack" a locked profile, there are a few legitimate, non-invasive methods that might show you what you are looking for without breaking the law or installing software.

Let’s explain why.

There is no such thing as a “Facebook locked profile picture viewer online.” Any website promising one is either trying to scam you, steal your data, or infect your device. The locked profile picture feature exists for a reason—to give users control over their image. If someone doesn’t want you to see their profile picture in full, respect their privacy.