WelcomePage: Report sizes of loaded images

It's so much nicer for the qml profiler to know the actual sizes of
images ...

Change-Id: Ib1c0d38b002a061e7cffe2619034e144e4d5f2cf
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>