<?php

add_filter( 'toolbox/helpers/sc_attr/type=image' , 'add_image_class_attr' , 10 ,1 );

// add a custom attribute named 'myclass'
function add_image_class_attr( $attr ) {
	$attr['myclass'] = '';
	return $attr;
}

add_filter( 'toolbox/helpers/set_acfimage_classes' , 'enable_custom_image_classes', 10, 2 );

// add the custom atrtibute 'myclass' to the $classes array
function enable_custom_image_classes( $classes , $attr ) {

	if (isset($attr['myclass']) && $attr['myclass'] !='' ) {
		array_push( $classes , $attr['myclass'] );
	}

	return $classes;

}