ChronoEngine has provided detailed instructions how to add refresh button for ImageVerification at ChronoEngine FAQs page. Here it is what they say:

In chronocontact.php look for:

if ( trim($paramsvalues->imagever) == 'Yes' ) {
  $imver = '<input name="chrono_verification" type="text"
    id="chrono_verification" value="">
  <img src="/'.$mosConfig_live_site
            .'/components/com_chronocontact/chrono_verification.php">';
}

if ( trim($paramsvalues->imagever) == 'Yes' ) {
    $imver_url = $mosConfig_live_site
        .'/components/com_chronocontact/chrono_verification.php?imtype='
        .$paramsvalues->imtype;
    $imver = '<script type="text/javascript">
        function RefreshImageVer(imgverform) {
            var randnum = Math.random();
            var newverimage = \''.$imver_url.'\'+randnum;
            imgverform.imgver.src=newverimage;
        }
        </script>
        <input name="chrono_verification" type="text" id="chrono_verification" value="">
            <img src="/'.$imver_url.'" name="imgver" id="imgver">
        <input type="button" value="refresh" onclick="RefreshImageVer(this.form);" />';
}

And presto, one refresh button ready to go. Tested in IE7 and Fx2 From notexa on 2007/09/02 02:35

Please, go to ChronoEngine FAQs page to read the full article.

If you use their code you will notice that the verification image is shown only after clicking on the Refresh button. The solution is to remove the slash from the 13-th row from the new code in the image source. Or replace the code:

            <img src="/'.$imver_url.'" name="imgver" id="imgver">

with:

            <img src="'.$imver_url.'" name="imgver" id="imgver">

Now the verification image will appear from the first time.