You would have to replace the <g:plusone> tag in the code with a custom link so you could add your own image like:
<a href="https://plus.google.com/share?url=<?php echo (((!empty($_SERVER["HTTPS"])) ? "https://" : "http://") . $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"]); ?>" >
<img src="path_to_your_image" alt="Google+" title="Google+"/>
</a>