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<?php
function display_custom_image_sizes( $sizes ) {
  global $_wp_additional_image_sizes;
  if ( empty($_wp_additional_image_sizes) )
    return $sizes;
  foreach ( $_wp_additional_image_sizes as $id => $data ) {
    if ( !isset($sizes[$id]) )
      $sizes[$id] = ucfirst( str_replace( '-', ' ', $id ) );
  }
  return $sizes;
}
add_filter( 'image_size_names_choose', 'display_custom_image_sizes' );