The cart on our demo page is exactly the same cart all our clients use, so they should all behave the same way; however, different Web browser programs may treat the alt-text differently -- some may show it as a hovering "tool tip" label, others only show it if the image itself fails to load (or if the user has disabled images), others make it a user preference. Its primary purpose is for visually-impaired users who may be using a screen reader, so the alt-text tells them what the button is for, if they can't see the image.
That said, you can change what the alt text says by adding the following to your View Cart code on every page, just before the "// -->" line in the standard View Cart code you get from Seller Admin (you can change "Click here for checkout" to say whatever you wish):
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#btnPP").attr("alt", "Click here for checkout");
}