Search your php code for anything that refers to: "CheckoutWizard_submit"
Then replace it with "CheckoutWizard_submit_x"
Once you change from a type="submit" to a type="image" it no longer submits as: CheckoutWizard_submit instead two values are submitted CheckoutWizard_submit_x and CheckoutWizard_submit_y based on the position of the image that was clicked to allow image mapping.