Hello Riccardo Scagnoli,
We checked your issues and found that you need to disable the featured image on single post. After that your desire image will appeared on that page. Please edit the post and scroll to down. In Other Options field you will get an options to hide the featured image for that particular single post. Or, if you want to hide it, you can hide it globally from Customize > Single Post Options > Feautred Image Options > Hide Featured Image. Have a look here on Screenshot.
Regards,
Acme Themes