A friend changed my daughters wallpaper on her iPad mini and now we can't find the picture to put it back? Any ideas?

All devices (iPads, iPhones, iPods) that you backup to that computer will have their backup stored under that folder : ~/Library/Application Support/MobileSync/Backup/ - if you backup multiple devices to that Mac you should have multiple folders directly under that Backup folder. iTunes generally only keeps one backup per device (I've only had more than when updating a device's iOS via the computer, when I've had pre- and post-update backups), that then gets overwritten/updated each time that you take a new backup. What is within each of those folders is the (backed up) content from that device.

A device's backup is only designed to be used by iTunes (for restoring onto a device), it's not designed to be accessed by people (so it's doubtful that you will be able to make any sense of what the folder's content), so the folder names will just be a sequence of numbers and letters, and the content within the folder will be similar (meaningless file names and content) - it's only if you restore the backup onto a device that you will be able to see what is in that backup.

To see which folder (if you are backing up multiple devices to that Mac) under that 'Backup' folder is for your iPad, you can go in iTunes > Preferences (command-comma) in iTunes, select the Devices tab, and control-click your iPad backup to get a popup which should have a 'show in finder' option, select that and Finder should open with its folder highlighted :