1. Go to Logbook page under Logbook menu.
2. Retrieve a closed Activity Report and open its details (by clicking on its line).
3. Scroll down to reach the bottom of the Report and the Comment section.
4. Use the available field to encode your comment. And use the toggle cursor beneath to set its visibility to Private (viewable only to users having the right to see private comments).
Want to add a picture to your comment?
Click on the 3 vertical dots on the right and then click on Add Media.
This will open a search prompt to find the file, validate to add the file to your comment.
5. And then click on Add Comment.
Your comment is now added with the others.