Paste it into a mobile browser (like Chrome or Safari) and change "www" to "m" in the URL (e.g., ://facebook.com Long-press the cover photo to bring up a menu and select "Download image" "Save image as"
Zoom in using Ctrl + + to make the image as large as possible. Use (Windows) or Cmd + Shift + 4 (Mac). download facebook cover photo of locked profile