Why is the background on the button and not on the main window (PyQt5)?

Question

PyQt

Good afternoon!
I'm trying to insert a background image into the main window of a windows application. I am using PyQt5 from Riverbank.
I inserted the picture like this:

self.setStyleSheet("background-image: url(x_pic.png);"
                   "background-repeat: no-repeat;background-position: center;")

The GUI did this:

class Interface(QWidget):

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

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

        start_button = QPushButton('Start', self)
        start_button.clicked.connect(run_convertor)
        start_button.resize(start_button.sizeHint())
        start_button.move(100, 100)
        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("background-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())

The background appears on the button, not the window. How to make the background not for the button, but for the main window?

Answer 1

Replaced QWidget with QMainWindow. At the same time, I made the button round and invisible (under the picture it is necessary).

The code

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.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')
        start_button.setStyleSheet("QPushButton {background-color: rgba(255, 0, 0, 0);"
                                   "border-radius: 100px;}")

        self.resize(1000, 500)
        self.center()
        self.setWindowTitle('jpgTabaki')
        self.setWindowIcon(QIcon('icon.png'))
        self.setToolTip('Window tip')
        self.setStyleSheet("QMainWindow {background-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())