My guess is the responsive nature of Sense would work against a nice deployment of one. You can insert sheet and app images if that kind of styling floats your boat but generally speaking I've always thought background images never enhanced the visualisation and were somewhat of a distraction if applied over zealously.
You can not have a background image to the entire dashboard in QlikSense and its because of the responsive behavior of the layout which is very beneficial. I totaly agree with Andy's thought that a background image has never improved the visual effect of a dashboard.
Background image could be a distraction for analyst/technician, but sometimes few stakeholder such as marketing and top management feels that unique visualization is somewhat important. Couple of times I used simple image like logo or icon placed in left upper corner as workaround for win-win solution
