Hello Stefano - I am not sure if this is something that is intended for this type of product. There is a significant distinction between self-service visualizations (Qlik Sense) and guided analytical applications (QlikView) - where QlikView might be more suited to your needs and does have this capability of adding a backgound images to a chart object.
With Sense, if you need to present the information with context and custom graphics - this "technique" I described in Data Storytelling might be the best option.
Also keep in mind there are best practices when defining visualizations, so please take that into consideraton as you design your app.
I'd like to inform you that there should be a wrong email address into you personal account on QlikView, as I am receiving notifications of your activity on the platform. Could you please take a look at your account and change the configuration? Thanks a lot! :-)
As Mike says, it is possible to do this in Storytelling, however, and you probably know this already (coming from a data visualisation perspective in QV11 and Sense), I would only look to add images to a scatter when the image has a direct link to the placement of the plots, for example a map or floor plan. I would strongly advise against any other "art work" being added when it has no link to the data itself.
I still see people adding images to the back of bar charts which is another BIG no no.