funtions.php
function give_linked_images_class($html, $id, $caption, $title, $align, $url, $size, $alt = '' ){
$classes = 'image_sc lightbox_img';
if ( preg_match('/<img.*? class=".*?">/', $html) ) {
$html = preg_replace('/(<a.*? class=".*?)(".*?>)/', '$1 ' . $classes . '$2', $html);
} else {
$html = preg_replace('/(<img.*?)>/', '$1 class="' . $classes . '" >', $html);
}
return $html;
}
add_filter('image_send_to_editor','give_linked_images_class',10,8);
Vê se resulta...