Comment 16 for bug 1373462

As the size of the image is not known in advance it seems its up to the app per http://qt-project.org/doc/qt-5/qml-qtquick-image.html

Note: Images are often the greatest user of memory in QML user interfaces. It is recommended that images which do not form part of the user interface have their size bounded via the sourceSize property. This is especially important for content that is loaded from external sources or provided by the user.