自己随便写个类继承QWidget，然后在这个控件上放上你的图片。
最后在mainwindow.cpp这种文件中实例化那个类，然后摆摆位置，layout一下就万事大吉了。
至于要置顶显示，请使用：
void QWidget::raise () [slot]
Raises this widget to the top of the parent widget's stack.
After this call the widget will be visually in front of any overlapping sibling widgets.
Note: When using activateWindow(), you can call this function to ensure that the window is stacked on top.
See also lower() and stackUnder().
