用image来提交form不想使用button提交form

<form method="post" action="formtest.html" target="_blank" name="formtest">
<input type="hidden" name="userid" value="userid">
<input type="hidden" name="username" value="username">
<input type="image" src="imagesubmit.gif" border="0">
</form>

<form method="post" action="formtest.jsp">
<input type="hidden" name="userid" value="userid">
<input type="hidden" name="username" value="username">
<input type="image" src="imagesubmit.gif" border="0">
</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>