FYI:
this "fix" of public function image() in class-twitter.php is working
/**
* Displays the image for Twitter
*
* Only used when OpenGraph is inactive.
*/
public function image() {
global $post;
if ( function_exists( 'has_post_thumbnail' ) && has_post_thumbnail( $post->ID ) ) {
$featured_img = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), apply_filters( 'wpseo_opengraph_image_size', 'medium' ) );
if ( $featured_img ) {
$img = apply_filters( 'wpseo_opengraph_image', $featured_img[0] );
echo "<meta name='twitter:image' content='" . esc_attr( $img ) . "'>\n";
}
} else {
$options = get_wpseo_options();
if ( isset( $options['og_frontpage_image'] ) && !empty( $options['og_frontpage_image'] ) )
$img = apply_filters( 'wpseo_opengraph_image', $options['og_frontpage_image'] );
echo "<meta name='twitter:image' content='" . esc_attr( $img ) . "'>\n";
}
}