I digged deeply:
when I upload or insert an image from Media Library in a Post or a Page (=with standard procedures) and Duplicate uploaded media to translations is checked, it makes a "copy" of the image in Media Library.
This way I can use one instance for the post in English, one for the one in Italian: the image is only one, but I have 2 instances available, and I can modify title, alt, description, etc, independently.
This doesn't happen with images uploaded by my custom field image into a Custom Post: it doesn't create another instance of the uploaded or inserted image. I have only one instance of the image, with its title, alt, etc. Only one for all languages.
Why this kind of images is seen differently by WPML and it doesn't "duplicate" them?
I have just written also in WPML forum (it is available only for who bought the plugin). Not an answer yet, I'm waiting, but in a top post they suggest this:
"If you're running into compatibility problems with other plugins or themes, the best way to fix them is to get the authors of those plugins / themes involved.
We work together with any author who's prepared to work with us.
Contact the other author and let us know. We'll create free accounts for them so that they can download WPML and participate in this forum.
We'll do everything that it takes to get WPML to work together with other plugins and themes. All we need is cooperation from the other side as well."
I report it as it is written: I don't ask you to do this because I guess you're very busy, and you are already so kind answering my silly questions.
If you have any idea to fix this issue, I'll appreciate your suggestions.
Thank you, as always.