To change the button you will need to edit some code in the skin files for OPC.  You will want to look for code that begins as <input type="button" .
In this case, we are changing the

<input type="button" value="{$msg.cart.get_shipping_rates|htmlspecialchars}" onclick="opcGetShippingRates()"/>

code to modify the get shipping rates button in opc.

Once you locate that line, you will want to comment it out using standard html comments <!-- -->  .
After you comment it out, right below, enter in the following line:
<img realsrc="<Path to desired image>" src="http://www.ixxocart.com/support/<Path to desired image>" alt="<desired alt tag text>" onclick="opcGetShippingRates()"/>