In the Qt 5.4 documentation of the QtQuick Item Item QML Type, the example of the grabToImage() method does not work.
The save() method does not exist.
But there is the SaveToFile() instead, see QQuickItemGrabResult.

// ...
    source.grabToImage(function(result) {
                           result.save("something.png"); // save method does not exist
                       });

Edit :
Snippet can be found in the Item QML Type : http://doc.qt.io/qt-5/qml-qtquick-item.html#grabToImage-method