As we reported that google stopped supporting Picasa and Said to use Google Photos so now google is Focusing on the Google Photos,
And it got update for its android app and the update includes many new features including ability to manually backup your photos and now the app version is 1.18.
Google Photos team wrote in a Google+ post:
“Save what you want, when you want with manual backup,”
Now update your Android Google Photos app, After that if you want to checkout manual backup feature select your photos that you want to backup then tap the three dot over the flow menu and there will be “Back UP Now” Option.