in skin1/customer/main/checkout.tpl look for
Code:
<TD nowrap><B>{$payment_methods[payment].payment_method} </B></TD>
<TD>{$payment_methods[payment].payment_details|default:" "}</TD>
if all you have is paypal and credit card as your payment methods then you can change the above to something like <TD colspan=2><img src="creditcardimages.jpg"></TD> and upload the image
if you have more than those 2 payment methods you will have to use if statements