You need to use the code from your View Cart button, not from the Add to Cart buttons.
The View Cart button code is consistent across your entire account and is available on the same screen where you get your Add to Cart buttons. This code contains the javascript which actually creates the customizable overlay cart and it is required for any customizations to work.
Your custom code is a little bit out of order and incomplete, it should read as follows:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#btnCC").attr("src", "http://www.blah-blah.com/wp-content/uploads/2013/10/CheckOut-Neue-cc.gif");
}
The above should be copied just before the //--> line in the View Cart button. It will replace our free checkout button with your own image, only in those cases where the cart total is $0.