How to scale a background image with aspect ratio with PyQt?

Question

PyQt

Good afternoon!
While it turned out to be done without maintaining the aspect ratio.

class Interface(QMainWindow):

    def __init__(self):
        super().__init__()
        self.main_window()

    def main_window(self):
        QToolTip.setFont(QFont('TimesNewRoman', 12))

        start_button = QPushButton('Start', self)
        start_button.setFlat(True)
        start_button.setStyleSheet("QPushButton {background-color: rgba(255, 0, 0, 0);"
                                   "border-radius: 100px;}")
        start_button.resize(200, 200)
        start_button.move(100, 100)
        start_button.setFont(QFont('Calibri', 42))
        start_button.clicked.connect(run_convertor)
        start_button.setToolTip('Button tip')

        self.resize(1000, 500)
        self.center()
        self.setWindowTitle('jpgTabaki')
        self.setWindowIcon(QIcon('icon.png'))
        self.setToolTip('Window tip')
        self.setStyleSheet("QMainWindow {border-image: url(x_pic.png);"
                           "background-repeat: no-repeat; background-position: center;}")
        self.show()

    def center(self):
        q_screen = self.frameGeometry()
        q_resolution = QDesktopWidget().availableGeometry().center()
        q_screen.moveCenter(q_resolution)
        self.move(q_screen.topLeft())

When I searched in English, I saw solutions with QWidget instead of QMainWindow, but my background disappeared altogether. Probably I didn't understand something. When I searched in Russian, I generally got some rubbish on the topic of analytics and conferences.

How can you keep the aspect ratio while scaling?

And more interesting. If you remove "background-repeat: no-repeat; background-position: center;", then the background also disappears. Why is that?