I created a child team for twenty ten, included the parents style.css & editor-style.css in the new css files and made some (minor) adjustments.
I also created a images/headers directory in my child theme.
Added headers with accompanying thumbnails (both of the same size as the parent's defaults). Now when I go into customisation and select Default, I still see the twenty ten default headers. Not my own...
What am I doing wrong here?