The widget window has a fixed white background, which is a good default setting.
And you can change the opacity, but this affects the entire widget, not the background image only.
It would be better to have the option to modify this, so the user can change the background color of the window or even have the option to turn the background color off (make it transparent).
It would also be nice to have the option to change the opacity of the background of the window, so the background image / color of the dashboard itself shines through.
Especially with designing custom infographics, this would be a tremendous improvement to be able to match this to the corporate identity of the company.