This snippet lets you edit the icon for the PayPal gateway in WooCommerce.

Woocommerce - change paypal icon

// Change the paypal icon
add_filter('woocommerce_paypal_icon', 'custom_woocommerce_paypal_icon');

function custom_woocommerce_paypal_icon( $url ) {
	$url = get_bloginfo('template_url')."/assets/img/payment-icons.png";
	return $url;
}