Zen Cart does not provide credit card images.
You may wish to contact your Merchant services for official images that you are permitted to use.
Once you have obtained the allowed images, upload your new gifs and replace the ones that are holding the place on the pics you see on your checkout page.
If you wish to override the defaults, the images should go here:
cc1.gif = Visa
cc2.gif = MC
cc3.gif = Amex
cc4.gif = Diners
cc5.gif = Discover
cc6.gif = JCB
cc7.gif = AustralianBankCard