Customizing twentyten — how to have different header images in a child theme?

This is likely a simple issue for someone with more WordPress midiclorians than I’ve got, but it’s perplexing me…

I’m attempting to customise the “twentyten” theme via a child theme, and want it to have a different selection of header images. I’ve edited the child’s “functions.php” to change the default image names to the new ones, and swapped out the images (and their thumbnails) in the theme’s directory. But when I attempt to use the new images via the admin panel, they aren’t displayed.

What am I missing?

TIA,

Sam