Welcome to Android Central! What directory are those image files in? Look for a file called ".nomedia" in that directory -- that file instructs the system to ignore that directory when scanning for images to add to the Gallery database. You could delete the .nomedia file, but it might be added back by Facebook. It's better to copy or move the photo files from that directory to either the /DCIM/Camera or /Pictures directory. Once they're moved, the system should recognize them and add them to the Gallery database.