Card icons not showing in WooCommerce checkout

Paste the following code in Appearance > Customize > Additional CSS:

/* display credit cards icons at checkout */
.woocommerce-account form .payment_methods label img, 
.woocommerce-checkout form .payment_methods label img {
    display: initial !important;
    max-width: 35px !important;
}

To keep PayPal icon hidden, paste the following code as well:

/* keep paypal icon hidden */
.woocommerce-checkout .wc_payment_method.payment_method_paypal img {
	display: none !important;
}

Source: https://theme.co/forum/t/credit-cart-icons-do-not-show-in-checkout-page-stripe-and-paypal/3079/6