It should be added that “highest quality” means highest length x width in pixels wins. I came across quite some covers which while being a few pixels larger in this respect provided a lower overall visual quality than my file embedded cover art (which is usually something around 1200 x 1200 px). But I understand that evaluating the real image quality is probably not as simple as just comparing size.
Try this discussion for some inspiration / ideas on how to extract images “back”:
A backup will restore things as they were at the time of the backup so you would get the images as you had set them. I’m pretty sure about this because I just successfully moved a library from one core to another and everything restored itself fine. 
(Don’t forget to backup your current library before you start playing around with library backups…)