In the process of trying to backup some pictures to an external hard drive on my girlfriend's macbook, I ran into a bit of difficulty getting to the photos in order to copy them to the hard drive. I did a search for photos and found them in the directory /users/NAME/pictures/iphoto library/originals/YEAR, but when I clicked through Finder to get there, double clicking the iPhoto library opened the iPhoto application. In order to open the /originals folder, I opened the terminal and typed:
open "/users/NAME/pictures/iphoto library/originals"
Then you can peruse by date (or backup everything from 2011 and 2012). If you know what year's pictures you want to see, you can type
open "/users/NAME/pictures/iphoto library/originals/2012" where 2012 is the year you want.