Adding labels to photos on iPad or iPhone

I don't believe iOS permits this. The simplest method to keep track of photos would be to store them in folders you label.

You can certainly let Apple know this is a feature you'd like to see --> Feedback - iPhone - Apple

iOS does not let you do this natively, however, there are third party apps in the iOS App Store that will let you write or type text on photos/images.

Do a search of the iOS App Store for these types of apps.

There are many.

Best of Luck to You!