I've figured this out. The problem wasn't in fact due to image files pixel alpha. That works just fine. It had to do with the QQuickView not being transparent. What I am doing is displaying a QQuickView as a borderless window to be used as a popup dialog and I want to see through to the background, where there are no objects in the qml file. (Think of a pop up dialog with rounded corners).
Here's what I'm doing:
	QQuickView view;
	// don't display an icon in the task bar
	// no border on the window
    view.setFlags(Qt::SubWindow | Qt::FramelessWindowHint);
	
	// make the view transparent so that the edges of the window,
	// where there is no image, show through to the background
    QSurfaceFormat surfaceFormat;
    surfaceFormat.setAlphaBufferSize(8);
    view.setFormat(surfaceFormat);
    view.setClearBeforeRendering(true);
    view.setColor(QColor(Qt::transparent));
	// load qml file and display
	view.setSource(file.qml);
	view.show();