Obviously, I know that I can do this… but my question was different.
I wanted the placeholder already inserted by default in the theme and only after changed with the right picture.
Why do I want this? Because I noted a bug in the theme and this could solve the problem.
Please note that even if I insert all pictures… when the page is loaded with a slow connection (for example from a mobile with a cellular connection) the page is loaded initially without images (because not completely loaded) and after the pictures appear. But in this way when the pictures have finished being loaded, the page is already formed and in this way, the pictures are inserted in a wrong grid.
I would like to solve this bug with the placeholder already in the grid.
I hope you have understood the point.