怎样用image来提交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()" alt="怎样用image来提交form" > 
</form>