Donwload and install Google’s Featured Photos Screensaver on your mac first and select that from your Change Desktop Background > Screensaver.
Run this command from your terminal.
nohup /System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background > /dev/null 2>&1 &
You can create shell or application out of this command and run it at the time of startup.
In order to run this script at startup, create an app with Mac Automator application as given below (Library > Run Shell Script)
Save that as an application and run it on startup as given below, goto System Preferences > Users & Groups.
And you will have all google+ wallpapers for your mac on startup.