I wanna add in a block configuration form an image field that can take several images, just like in a node form, with all the drag'n drop stuff, etc. Any advices or solutions?
The solution is an image field configured in a custom block type.
Block forms are configuration and don't support image fields. Custom block types are content and they support the same field types as nodes.
If you can't do what you want with a custom block type, you can create your own block plugin which uses a block content entity.