Remove lightbox image sizes generated by OptinMonster.

gistfile1.php

<?php
add_filter( 'intermediate_image_sizes', 'tgm_om_filter_image_sizes' );
function tgm_om_filter_image_sizes( $sizes ) {

	$om_sizes = array( 'optin-monster-lightbox-theme-balance', 'optin-monster-lightbox-theme-bullseye', 'optin-monster-lightbox-theme-case-study', 'optin-monster-lightbox-theme-clean-slate', 'optin-monster-lightbox-theme-transparent' );

	foreach ( $sizes as $i => $size ) {
		if ( in_array( $size, $om_sizes ) ) {
			unset( $sizes[ $i ] );
		}
	}

	return $sizes;

}