I never used the twenty ten theme so I really don't know what featured image are you talking about, but I did some research and it seems it's a fucking mess, quite unexpected since you would expect a flawless theme. Anyway, just do this:
Code:
if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150, true ); // Normal post thumbnails
add_image_size( 'myfeatured', 300, 168 ); // Permalink thumbnail size
}
and then in your theme's single.php and index.php (ie under the title of the post)
Code:
<?php the_post_thumbnail('myfeatured'); ?>