Tagging people in Google+ photos - on a desktop
You can tag people from an album by licking on the Tag people on top right:
When you click on "Tag people" then the system searches for all human faces in those photos and also gives suggestions from Google+. One can tag people in any image, also screenshots, just by clicking on the "Tag people" and adding a profile or Page name.
NOTE: If you tag someone in a photo, they get to see all other photos in that album.
Tagging someone in just one image:
When you click open a photo in the lightbox view, on top you have some choices:
There you can also see "Tag people".
Tagging someone automatically is not always possible: It depends on the tagged person's settings.
More about tagging from Google:
I hope this helps.