void MainWindow::paintEvent(QPaintEvent *)
{
    qDebug() << "paintEvent";

    QPainter painter(this);

    //上半部分背景
    QPixmap banner(":/login/banner_5");
    banner.setDevicePixelRatio(2.0);
    painter.drawPixmap(0, 0, banner);

    //下班部分背景
    QPixmap bg(":/login/bg_login");
    bg.setDevicePixelRatio(2.0);
    painter.drawPixmap(0, banner.height()/2, bg);
}

Qt MainWindow 背景图片