Ok, after a while, I finally found what was the problem. I was displaying the picture like this:
w_vue, h_vue = self.vision.width(), self.vision.height()
self.current_image = QtGui.QImage(path_mosaic)
self.pixmap = QtGui.QPixmap.fromImage(self.current_image.scaled(w_vue, h_vue,
QtCore.Qt.KeepAspectRatio,
QtCore.Qt.SmoothTransformation))
w_pix, h_pix = self.pixmap.width(), self.pixmap.height()
self.scene = QtGui.QGraphicsScene()
self.scene.setSceneRect(0, 0, w_pix , h_pix - 10)
self.scene.addPixmap(self.pixmap)
self.vision.setScene(self.scene)
While the pixmap declaration should have been this, simply: self.pixmap = QtGui.QPixmap(self.current_image)
Nothing to do with the QGraphicsView.