iPhoto update issue

I have a Macbook Pro and an iMac, both with OS X 10.8.5. On the Macbook Pro I have iPhoto 9.5.1 and on the iMac I have iPhoto 9.4.3. Due to the size of my photo library (47,000 pics), I moved my library off my Macbook to an external hard drive. Now that it's on the hard drive, I can access it from my Macbook, but not from my iMac. I can't find an update available to get my iMac updated with iPhoto 9.5.1. Why doesn't it show up in the updates?