How do I add a tag for a photo (not album)

Poirot

I have figured out how to add tags to albums, but don't know how to add tags to photos, although it seems obvious that it must be possible because there is a tag holder below every photo.
Where and how do I add tags, or have I forgotten some option I have not found yet?

Thanks

acrylian

On the backend for each image (click on more fields below).

Poirot

Right! I had done that a few times and looked through the fields but none seemed to be for tags. I first still did not see it, but the tag list appeared now on the right. It's been there all the time, I just did not see it.

Thanks!!!