Use QGraphicsPixmapItem:
plt1 = pg.PlotWidget(w)
img = pg.QtGui.QGraphicsPixmapItem(pg.QtGui.QPixmap(fileName))
plt1.addItem(img)
# depending on your preference, you probably want to invert the image:
img.scale(1, -1)
# OR invert the entire view:
plt.invertY(True)