Welcome qtLearn071,
This has been answered many times on this board so a quick search probably would of provided you the results your were looking for.
There are a couple of ways of doing this. Either through the background role or using style sheets of the main window frame work.
(e.g. mainwindow->setStyleSheet( "image: url( filename );" ); // note: filename could be from qt resource file or absolute path/filename. )