Most "viewer" tools operate using one of two methods:
and replace it with one of the following:
Several third-party websites act as a viewer. You paste the Facebook profile URL, and they scrape the full-size image for you. While convenient, use these with caution.
If you see a blue default silhouette, it means the user has a temporary avatar (like a character from a game) or the ID is wrong. For most real users, this method pulls the original upload.