In recent years, Facebook has introduced a “Profile Picture Guard” feature—commonly referred to as a “locked” profile picture—to help users protect their images from being downloaded, shared, or screenshotted by strangers. The feature adds a blue border and shield icon to the photo, making it difficult for anyone outside the user’s friends list to view the full image in high resolution.

: For more on how these protections work, you can visit the Facebook Help Center regarding locked profiles . Warning Regarding Online "Viewers"

If you’ve spent any time on social media, you’ve likely seen a friend’s profile picture with a small padlock icon and a blue border. This is Facebook’s —a security feature designed to prevent strangers from downloading, sharing, or zooming in on your photo.

Here is the technical explanation: