QImage image_one, image_two;
...
QImage image_sum( qMax( image_one.width(), image_two.width() ), image_one.height() + image_two.height(), image_one.format() );
image_sum.fill( 0 );
QPainter p( &image_sum );
p.drawImage( 0, 0, image_one );
p.drawImage( 0, image_one.height(), image_two );
p.end();