pixmap.loadFromData(data)
self.previewImage.setPixmap(pixmap)
self.previewImage.show()
self.updater.start()

def main():
    app = QtGui.QApplication(sys.argv)
preview = Preview()
sys.exit(app.exec_())
if __name__ == '__main__':
    main()
How to show and update a bitmap FAST in Python?