Where does Photos store iCloud photos shared with me?
I found the local storage location for iCloud shared images.

cd "/Users/simon/Library/Containers/com.apple.cloudphotosd/Data/Library/Application Support/com.apple.cloudphotosd/services/com.apple.photo.icloud.sharedstreams/assets"


Replace simon with your username.

Of course, the ridiculous obscurity of this path means they'll probably change it again, but this is what it is as of OS X 10.12.5 and Photos Version 2.0 (3150.4.120).