If your image isn't transparent as it is and you want it to be, you can do something like this:
QLabel *l = new QLabel(this);
QImage image(":/img/myimage.png");
QPainter p;
p.begin(&image);
p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
p.fillRect(image.rect(), QColor(0, 0, 0, 50));
p.end();
l->setPixmap(QPixmap::fromImage(image));