Hi,
another stupid question... do you kno if it possible, with a plugin I guess, choose to use an image as the cover of the article, not from our image archive, but from an external sourcce, therefore inserting the url of the image?
Thanks guys

You can link it directly from the source in your editor <img src=".." alt=".." />

Therefore, if I understand right... I do not have to insert the cover image from the "image" tab, but I have to insert it directly in article content in the "content" tab. Thus the first image inserted in the article, automatically becomes the article cover. Right?
For sake of clarity, attached a screenshot of the "image" tab (in Italian).

You can use an extrafield to store the external URL and use that field instead of the image,
You will need to override the item.php file -> getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates and alter the image's code a bit.

A rough example can be found here:
github.com/kricore/Advanced-templating-with-K2/blob/master/_inc/cheatsheet.php#L102-L118