The code added to the page by the eCommerce toolkit is a collection of form elements. One of thoser form elements is the submit button, the src attribute points to the image to display:
<input type="image" name="submit" border="0"
src="https://www.paypal.com/en_US/i/btn/btn_buynow_LG.gif"
alt="PayPal - The safer, easier way to pay online">
you can change the src attribute to point to your custom image.