Add a icon to the basket

yes, but done via css

Can your provide me with the code needed? I have my own icon ready.

quick & dirty – paste into “Custom CSS”

.spreadplugin-checkout-link:before {
  content: "";
  float: left;
  width: 32px;
  height: 32px;
  margin: 0 5px 0 0;
  background: url('https://png-1.findicons.com/files/icons/1672/mono/32/shoppingcart.png') no-repeat 0 0;
}

or

.spreadplugin-checkout-link:before {
  content: url('https://png-1.findicons.com/files/icons/1672/mono/32/shoppingcart.png');
  width: 32px;
  height: 32px;
  margin: 0 5px 0 0;
}