Yes, simply replacing the src= URL with a custom image URL in your standard E-junkie button code (as shown in your example above) should work fine. However, looking at the custom button image URL you have specified there, you may wish to consider whether buyers would realize they must click your book cover image to place an order. Typically a purchase button should at least have some "call to action" text telling buyers what to do and what will happen if they click the button -- e.g. "Add to Cart" or "Buy Now" or "Place Order", etc.