Hmm, missed this one. There was another like it a day or two ago.
Anyway, this is the method when using the FaceBook app. I have not looked into using the website through Safari. I may get around to that later, but it's not something I normally need to do.
So:
1. Post your picture to FaceBook by whatever method you normally use.
2. Find the picture and tap on it until it opens full screen.
3. Tap on the Tag icon at the lower left (only appears for your photos).
4. Tap on the face of the person you want to tag.
5. Start typing their name (FaceBook name if not the same as their real name)
6. Choose the person from the list of suggestions that appears.
6.1. If the person you want to tag is not a FaceBook friend, keep typing until you finish the name, then tap Done on the keyboard.
Repeat 3 - 6 until done tagging.