Gallery auto actions

How to modify the privacy auto actions for galleries to display a unique media (for example a png file with an inscription "this content is private" saved in the directory "www / images / ...") each time a media (photo or video) is not visible to the viewer user instead of "skipping" the media ?

There's no functionality for display overrides of content other than Fields or entire Tabs. You won't be able to have a display override for media like that. It maybe something available in the future, but typically it's bad UX to show something they can't access.

Bad news !
I have an author who has 24 media including 3 public and 21 accessible by subscription, some visitors refuse to subscribe to his profile because they think he has only 3 media published ... and they do not see that he there are 21 other media to which he could subscribe.
Many sites offering subscription media replace copyrighted media with a single post or png.
I'm disappointed that gallery auto actions don't allow this.

That is simply outside the scope of what CB Gallery is designed to provide. It's not possible for CB Privacy to override CB Gallery media display like that. It maybe something supported in a future release, but for now it's just not possible and would require a significant rewrite to template files as well as their trigger behavior.