I think you can try to use this code...
<form id="frmCaptcha" name="frmCaptcha">
<table>
<tr>
<td align="left">
<label for="captcha">Captcha</label>
</td>
<td>
<input id="txtCaptcha" type="text" name="txtCaptcha" value="" maxlength="10" size="32" />
</td>
<td>
<img id="imgCaptcha" src="create_image.php" />
</td>
</tr>
<tr>
<td> </td>
<td>
<input id="btnCaptcha" type="button" value="Captcha Test" name="btnCaptcha"
onclick="getParam(document.frmCaptcha)" />
</td>
</tr>
</table>
<div id="result"> </div>
</form>