How to replace the default gateway card brand images?

To change the default gateway card icons you need to put this into the functions.php file of your theme:

function gb_wc_ocu_replace_images( $icon, $gateway_id )
{
 if( $gateway_id === 'ocustripe' )
 {
  $icon = '<span><img src="url/to/your/icon.png" /></span>';
 }

 return $icon;
}
add_filter( 'woocommerce_gateway_icon', 'gb_wc_ocu_replace_images', 10, 2 );