Code to edit the image size in invoices - By WebTofee

function.php

<?php
add_filter('wf_pklist_add_custom_css','wt_pklist_alter_product_image_col',10,2);
function wt_pklist_alter_product_image_col($custom_css,$template_type)
{
	if( 'invoice' === $template_type )
	{
		$custom_css.='.wfte_product_image_thumb{ max-width:none !important; max-height:none !important; width:100px !important; }';
	}
	return $custom_css;
}