Adds credit card logos and a PayPal logo to the "Select Payment Method" box on membership checkout.

my_pmpro_payment_method.css

/* CSS Document */
.pmpro-checkout #pmpro_payment_method span {
	display: inline-block;
}
.pmpro-checkout #pmpro_payment_method span a {
	background-position: bottom left;
	background-repeat: no-repeat; 
	background-size: contain;
	border: none;
	display: inline-block; 
	font-size: 16px;
	font-size: 1.6rem;
	padding-bottom: 80px;
	width: 90%;
}
.pmpro-checkout span.gateway_authorizenet, 
.pmpro-checkout span.gateway_braintree,
.pmpro-checkout span.gateway_cybersource,
.pmpro-checkout span.gateway_paypal,
.pmpro-checkout span.gateway_stripe, 
.pmpro-checkout span.gateway_twocheckout {
	width: 60%; 
}
.pmpro-checkout span.gateway_paypalexpress {
	width: 35%;
}
.pmpro-checkout span.gateway_authorizenet a, 
.pmpro-checkout span.gateway_braintree a,
.pmpro-checkout span.gateway_cybersource a,
.pmpro-checkout span.gateway_paypal a,
.pmpro-checkout span.gateway_stripe a, 
.pmpro-checkout span.gateway_twocheckout a {
	background-image: url(../images/cc-horizontal.jpg); 
}
.pmpro-checkout span.gateway_paypalexpress a {
	background-image: url(../images/paypal.jpg);
}

@media (max-width:768px) {
	.pmpro-checkout #pmpro_payment_method span {display: block; width: 100%; }
}

There is an updated version of this code recipe here: https://gist.github.com/kimcoleman/58dc2653bde9156d98dc3b439f4faa2b