Thanks,
first of all I realized that the bounding rect of the QGraphicsPixmapItem stays the same size, even if it has been scaled. I don't know if that's a problem as the boundingRect in that case is bigger than the Scene/View. The reason why my program is stuck is the function static bool EqualRegion(...) of qregion.cpp, but I can't really find the reason for this call, how to avoid it etc, but it seems to me that there is a strong relation between Image-Size and Scaling / Displaying-Performance if the Image is transparent, which is the case.
Daniel