How can I image my Macbook Pro's hard drive prior to sending to Apple for repair?
I would choose either SuperDuper or Carbon Copy Cloner. Make a clone of your hard drive that is a bit-perfect bootable copy. You can either use an external drive or one of those docks that you pop bare drive into. 

SuperDuper costs just under $30. Carbon Copy Cloner is donationware. Either one is great. But I like CCC because it can schedule cloning sessions on a regular basis.