More technical users may use the "Inspect" tool (F12) in their browser to search for image source links (often under the "Sources" or "Network" tabs), though this typically only reveals what is already loading on the screen. Risks and Privacy Considerations
This is the obvious, non-technical solution. The "Lock Profile" feature only locks out non-friends . If the user accepts your request, you will see their full timeline, photos, and posts (subject to their individual post privacy settings). fb locked profile cover photo viewer
Even on a locked profile, the cover photo and profile picture remain in thumbnail form. However: More technical users may use the "Inspect" tool