怎样用image来提交form

<script>

function formsubmit(){

document.formtest.action="formtest.jsp";

document.formtest.submit();

}

</script>

<form method="post" name="formtest">

<input type="hidden" name="userid" value="userid">

<input type="hidden" name="username" value="username">

<img src="imagesubmit.gif" border="0" onclick="formsubmit()">

</form>