How to set a Google Plus photo as a contact-picture?
I know there are tons of UI differences between the Nexus and Samsung Galaxy S4. However, here's what works for the S4, in case it leads you in the right direction.

From the Google+ slide menu, go to "Photos" and tap the desired photo. Once open, tap the phone's hardware menu button, then tap "Set as..." --> "More" --> "Contact photo" and select the desired contact.

Apologies if your Nexus doesn't have the "Set as..." menu option.