1. See this thread to add payment logos to the payment details page.
http://forum.x-cart.com/showthread.php?p=131711#post131711
THEN -- in special instructions, add something like this:
<img src="path/to/creditcards-logo.gif" /><br />VISA, AMEX, Discover, Mastercard
2. See this thread to add smarty variables to the payment details.
http://forum.x-cart.com/showthread.php?t=46254