Thank You for your tipps. However the picture does not show up. I tried to change the echo command to read the contents of the post meta key “thumbnail_html”
// Featured Image
// =============================================================================
function x_featured_image( $cropped = '' ) {
$stack = x_get_stack();
$fullwidth = ( in_array( 'x-full-width-active', get_body_class() ) ) ? true : false;
if ( has_post_thumbnail() ) {
if ( $cropped == 'cropped' ) {
if ( $fullwidth ) {
$thumb = get_the_post_thumbnail( NULL, 'entry-cropped-fullwidth', NULL );
} else {
$thumb = get_the_post_thumbnail( NULL, 'entry-cropped', NULL );
}
} else {
if ( $fullwidth ) {
$thumb = get_the_post_thumbnail( NULL, 'entry-fullwidth', NULL );
} else {
$thumb = get_the_post_thumbnail( NULL, 'entry', NULL );
}
}
switch ( is_singular() ) {
case true:
printf( '<div class="entry-thumb">%s</div>', $thumb );
break;
case false:
printf( '<a href="%1$s" class="entry-thumb" title="%2$s">%3$s</a>',
esc_url( get_permalink() ),
esc_attr( sprintf( __( 'Permalink to: "%s"', '__x__' ), the_title_attribute( 'echo=0' ) ) ),
$thumb
);
break;
}
} else {
$thumb = get_post_meta( get_the_ID(), 'thumbnail_html', true );
switch ( is_singular() ) {
case true:
printf( '<div class="entry-thumb">%s</div>', $thumb );
break;
case false:
printf( '<a href="%1$s" class="entry-thumb" title="%2$s">%3$s</a>',
esc_url( get_permalink() ),
esc_attr( sprintf( __( 'Permalink to: "%s"', '__x__' ), the_title_attribute( 'echo=0' ) ) ),
$thumb
);
break;
}
}
}
I also added the function from classes.php to the x-child/functions.php and added an “or” to the “has_post_thumbnail()” part to enable the “thumbnail_html” meta key.
if ( ! function_exists( 'x_post_class' ) ) :
function x_post_class( $output ) {
switch ( has_post_thumbnail() || metadata_exists( 'post', get_the_ID(), 'thumbnail_html' ) ) {
case true :
$output[] = 'has-post-thumbnail';
break;
case false :
$output[] = 'no-post-thumbnail';
break;
}
return $output;
}
add_filter( 'post_class', 'x_post_class' );
endif;
Now each post that has a “thumbnail_html” meta key will display a placeholder, but the code for the thumbnail (or the echo command above, or even "echo '123'; )
Any ideas? 
