如何在 PyQt4 中相对于标签大小的变化保持按钮不变

I have an image as my label in a PyQt4 Program. This program also contains some push buttons. Now, when I maximize my window, though the label gets expanded but the push button retain their place. I want that the push buttons should maintain their relative position with respect to the label, even when maximized.

我的代码如下:

class Ui_MainWindow(object):
   def setupUi(self, MainWindow):

       self.centralwidget = QtGui.QWidget(MainWindow)
       MainWindow.setCentralWidget(self.centralwidget)
       lay = QtGui.QVBoxLayout(self.centralwidget)
       self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
       self.label = QtGui.QLabel(self.centralwidget)
       self.label.setText(_fromUtf8(""))
       self.label.setPixmap(QtGui.QPixmap(_fromUtf8("Capture.PNG")))
       self.label.setObjectName(_fromUtf8("label"))
       self.label.setScaledContents(True)
       lay.addWidget(self.label)
       self.Langdon = QtGui.QPushButton(self.centralwidget)
       self.Langdon.setGeometry(QtCore.QRect(290, 90, 75, 23))
       self.Langdon.setObjectName(_fromUtf8("Langdon"))     
       self.ChainLakes = QtGui.QPushButton(self.centralwidget)
       self.ChainLakes.setGeometry(QtCore.QRect(50, 290, 85, 23))
       self.ChainLakes.setObjectName(_fromUtf8("ChainLakes"))

   if __name__ == "__main__":
      import sys
      app = QtGui.QApplication(sys.argv)
      MainWindow = QtGui.QMainWindow()
      ui = Ui_MainWindow()
      ui.setupUi(MainWindow)
      MainWindow.show()
      sys.exit(app.exec_())

我应该如何修改我的代码??