Facebook introduced profile picture guard to protect users profile images. If you are not aware of that, then it’s a kind of features to prevent misuse of facebook users profile images. If users activated this option on their Facebook profile image, then other people can’t tap Share, Send in the message, Share externally, Zoom/View full size of the image or Download from your current profile picture on Facebook.
A small blue shield icon will appear below the users profile if they activated this facebook profile image guard feature. You can read more about this feature here.
Many of the users thought that this feature would prevent other users from downloading their profile images from facebook. But the fact is NO. It won’t stop others from downloading your profile picture. With a small little trick, any users can easily download facebook profile guarded image from facebook easily.