How to properly set a background image of a custom widget

[franco.amato]

Hi forum,
I have a custom widget, showing the output of a video camera ( after some calculations ) and I would display a background png image.
I tried with the

myWidget->setStyleSheet("background-image: url(:/screen.png);");

myWidget->setStyleSheet("background-image: url(:/screen.png);");

To copy to clipboard, switch view to plain text mode 

with no results.
Should i set it manually in the paint event?
Regards