If you edit the CAPTCHA using the properties floater, you will see a checkbox for "Include a Refresh Button". It will add the refresh button.
You could probably just add one manually with this:
<img src="webassist/captcha/images/refresh.png" height="18" onClick="document.getElementById('capt1').src+='&ref=1'">
make sure 'capt1' is updated to match the id of the CAPTCHA image on your page.