initialization of the imageSrc property to be *before* we call the
superclass. This pullup (Closes#1475), and is the last thing I'm going
to touch before the 2.6 release. svn merge -r6749:6751 trunk branch.
git-svn-id: http://svn.openlayers.org/branches/openlayers/2.6@6752 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf