How to set all the labels transparent using "QLabel{background:transparent}" ?

[sunRainPenguin]

qApp->setStyleSheet("QLabel{background:transparent}") ;
lab2->setStyleSheet("background:transparent");
When I use these two statements above, there are different effects to the labels:

label.jpg

The first statement can't make the labels transparent. And I need to know that how to make all the labels transparent when there's a background-image on the QMainWindow?