There doesn’t seem to be any way to do this using the pre-set “customizing” panel for the background image. Using the buttons on that panel merely repeat the image over and over rather than across the top, where I want it.
I have managed to find the selector for body.custom background in Firebug and pasted that into my Child Theme, but it doesn’t update the theme.
I have changed the “version” of my child theme to 2.0.4 but it doesn’t seem to connect.
Completely at a loss as to how to fix this now that I’ve updated the Graphene Theme.