Draw an image as background at the erase background time (and not paint time)

[tbscope]

First things first: without doing some magic voodoo, Qt can only paint on widgets inside the paint event.

With that in mind, you know that you will always need to paint the background from within the paint event.

Flickering of the background can be solved by making use of double buffering.
http://doc.trolltech.com/qq/qq06-flicker-free.html