You should better use a custom hooked function in woocommerce_email_customer_details
action hook for example, this way:
add_action( 'woocommerce_email_customer_details', 'custom_image_near_email_footer' , 50, 4);
function custom_image_near_email_footer( $order, $sent_to_admin, $plain_text, $email ) {
// For customer order notification with "completed" status
if ( 'customer_completed_order' == $email->id )
echo '<div style="text-align:center;">
<img src="'.home_url( "wp-content/uploads/2017/05/myPic.jpg" ).'" alt="myPic"/>
</div>';
}
You could also use woocommerce_email_footer
action hook instead which have only one argument ($email
argument).
Code goes in function.php file of your active child theme (or theme) or also in any plugin file.
This code is tested and works on WooCommerce version 2.6.x and 3.0+