Is it possible for a user to upload images to the image/video grid using a user form and password?
Hi @stefanobianchi and welcome onboard! 
The best way to look at Sitely is that it is a localhost (on your Mac) CMS - Content Management System. If your client gave you the images then you’d be able to update the images. On the other hand if your client had a copy of the Sitely project file say via iCloud/Share then you can teach them how to go about it themselves.
To keep everything synced your client would update the Sitely project file in the iCloud so you can take a copy of it, keeping your Sitely project file up to date on your end.
Non è questo che mi serve voglio che un user sia in grado di fare upload di foto in una galleria già esistente senza il mio intervento
This is not what I need, I want a user to be able to upload photos to an existing gallery without my intervention
Yes and I mentioned that in my reply.
For them to update the Image/Video Grid they will need your Sitely project file, and you giving them a helping hand in how to do it.
Hi @stefanobianchi,
Sitely websites are (currently) static, so no amount of user interaction can change the site as it is seen by other visitors.