问个绘图的有关问题

问个绘图的问题
为什么在paintEvent里写

C/C++ code

QPalette p = palette();    QPixmap img(":/images/1.jpg");    p.setBrush(QPalette::Window, QBrush(img));    setPalette(p);

C/C++ code

QPalette p = app.palette();    QPixmap img("/home/shana/1.jpg");    p.setBrush(QPalette::Window, QBrush(img));    app.setPalette(p);