This help page explains how to customize the overlay-style E-junkie cart that appears "inside" your page, including how to replace the Your Shopping Cart icon with a custom logo:
http://www.e-junkie.com/ej/help.custom-cart.htm
If you're actually referring to the FatFreeCart.com service, please bear in mind that we provide that on a strictly do-it-yourself basis with no support whatsoever (that's a major reason it's free :^), so you're expected to figure it out and troubleshoot any issues you may have with that on your own. If you decide to upgrade to the paid E-junkie cart service (aka "FatFreeCart Pro"), that does of course include support with your subscription. That said, I think FatFreeCart should be able to use the same customization technique as the E-junkie Cart.
