The TwentyTen theme has an annoying feature which causes the image in your site header to be replaced by a post's Featured Image. I have no idea what they were thinking, other than that the featured image wasn't used anywhere else in the theme, so perhaps they should use it to override the image in the header.
Well that's a bad idea. Even though the theme doesn't use it, sites like Facebook will use the featured image as the thumbnail when a post is shared. It just doesn't make sense to change your site header according to the featured image.
In order to fix this, you have to edit your theme's header.php, preferably by creating a child theme. Search for these lines:
// Check if this is a post or page, if it has a thumbnail, and if it's a big one if ( is_singular() && current_theme_supports( 'post-thumbnails' ) && has_post_thumbnail( $post->ID ) && ( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) && $image >= $header_image_width ) : // Houston, we have a new header image! echo get_the_post_thumbnail( $post->ID ); elseif ( get_header_image() ) :
And change them to:
// Always use the header image; dont put the featured image in the header! if ( get_header_image() ) :
If you use MailPress, note that it attempts to follow suit; and even worse, in doing so it triggers a bug that prevents posts with a featured image from being sent at all. See this article on how to modify it too.