To mark images as private (i.e. if your customer does not want an image to be shared when they share the gallery) your customer can click on the eye on a particular image:
For example the gallery looks like this:
We now want to hide the first picture. To do this, we click on the eye.
Then we click on "Share images" in the menu on the left:
Now we select, for example, that we want to share all images:
We DO NOT select "Share hidden images". Now we click on "Copy link":
When you now open the link, the marked image is hidden: