functions.php

<?php

#functions.php
function chrisypoo_add_custom_types( $query ) {
    if( is_category() && empty( $query->query_vars['suppress_filters'] ) ) {

        $query->set( 'post_type', array(
        	'post', 'screenshot'
	     ));

        return $query;
    }
}
add_filter( 'pre_get_posts', 'chrisypoo_add_custom_types' );