Ah, no worries - it's simple...
Provided you have made a backup or have automatic backup enabled, connect a USB drive to restore (again, if you're on a friend's computer, login and disable cloud sync!), then right-click on the drive and select Device Library Restoration:
Next, select the image you want to restore (I only have the one as a demo):
Then click Restore to device. You'll see a confirmation window:
Click OK to complete the restoration.