Your client opens the comment window
On a computer, the speech bubble appears on a bar in the corner of the photo when your client hovers over it. On a phone, it's at the bottom of the enlarged view. Your client doesn't have to select a photo to comment on it.

They type the comment
A comment can be up to 500 characters, and the field won't take more. That's enough for a clear note about one photo.
One click saves it
When your client clicks Save, the window closes and the comment saves in the background, without reloading the page. They can come back to edit it, or delete it by saving an empty field.
You read the comments in the panel
In the gallery view, your client's comment shows up with the photo it's about. So when you edit, the note is exactly where you need it. Once your client has written something, the sort menu gets a Commented first option.