twentyten changing set_post_thumbnail_size

I am creating a child theme and have changed most of the setting by remove_filter and remove_action calls, and resetting the values, changed the header size so admin can pick up a new header, and added my own sidebars just fine.

I do not want the page and post header function, and I want a different post thumbnail size.

I have now been trying to reset the set_post_thumbnail_size() for about 4 hours, including Google searches, and to add new values in the child themes functions.php but it is still using the twentyten headers width and height for the feature thumbnails:

I have tried to remove the old values with:
remove_theme_support('post-thumbnails');

And replacing them with:

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 200, 160, true );

And tried overriding it with:

if(!function_exists('set_post_thumbnail_size'))
{set_post_thumbnail_size( 200, 160, true );}

As per the documentation added in a function which is added by:
add_action( ‘after_setup_theme’, ‘twentyten_custom_setup’ );

I don’t have much hair left so any help!

David