bool QGraphicsItem::collidesWithItem ( const QGraphicsItem * other, Qt::ItemSelectionMode mode = Qt::IntersectsItemShape ) const [virtual]
as a QGraphicsItem you can pass QGraphicsRectItem(scene->sceneRect())
It means that you create graphics item with sboundary same as of graphicsscene has and then you check if your pixmap icollides with this RectItem