class ImageViewer(QMainWindow)
        ...
        self.imageLabel = QLabel()
        self.scrollArea = QScrollArea()
        self.scrollArea.setWidget(self.imageLabel)
        ...

self.pixmap = QPixmap.fromImage(image)
    self.imageLabel.setPixmap(self.pixmap)

def mousePressEvent (self, eventQMouseEvent):
    self.originQPoint = eventQMouseEvent.pos()
    self.currentQRubberBand = QRubberBand(QRubberBand.Rectangle, self)
    self.currentQRubberBand.setGeometry(QRect(self.originQPoint, QSize()))
    self.currentQRubberBand.show()

def mousePressEvent (self, eventQMouseEvent):
    self.originQPoint = eventQMouseEvent.pos()
    self.currentQRubberBand = QRubberBand(QRubberBand.Rectangle, self)
    self.currentQRubberBand.setGeometry(QRect(self.originQPoint, QSize()))
    self.currentQRubberBand.show()