Add custom payment method icons to Easy Digital Downloads

gistfile1.php

<?php
/**
 * Register the payment icon
 */
function pw_edd_payment_icon($icons) {
    $icons['url/to/your/image/icon.png'] = 'Name of the Payment Method';
    return $icons;
}
add_filter('edd_accepted_payment_icons', 'pw_edd_payment_icon');