add_filter( 'post_thumbnail_html', 'nelioefi_replace_thumbnail', 10, 5 );
function nelioefi_replace_thumbnail( $html, $post_id, $post_image_id, $size, $attr ) {
    if ( uses_nelioefi( $post_id ) ) {
        $image_url = get_post_meta( $post_id, '_nelioefi_url', true );
        global $_wp_additional_image_sizes;

        if ( is_array( $size ) ) {
            $width = $size[0];
            $height = $size[1];
        }
        else if ( isset( $_wp_additional_image_sizes[ $size ] ) ) {
            $width = $_wp_additional_image_sizes[ $size ]['width'];
            $height = $_wp_additional_image_sizes[ $size ]['height'];
        }

        $replaced_image = str_replace("/s0/", "/w".$width."-h".$height."-c/", "$image_url");
        $html = sprintf(' <img src="%s"> ', $replaced_image);
        }
    return $html;
}