Make sure to initialize the image. By default it's full of uninitialized pixels, and QGraphicsScene::render() will blend on top (source-over composition). When your app starts, chances are when you create the first image, it's going to be blank because the OS blanks those bits. Second time however, the image is likely to contain junk from the last time you rendered.

See QImage::fill(). Don't know if that will solve the problem but it's worth a shot...