Indeed this is more compact, I moved to this approach.

Now I have one additional problem that seems really strange to me. When I run the application in my computer, where I have qt installed, it works drawing the background image in the mainWindow and the transparent windows on top of it.

I then create a directory including the QtCore4.dll, QtGui4.dll and the executable and try it on another computer with windows xp. Everything works but the background image is not displayed.
The very same directory works with the background image dispalyed on my computer though.

I have checked for additional dependencies using the Dependency Walker and it only needs those two dlls from Qt.

Do you have any idea of what could be the problem?