ปัญหาการใช้รูปภาพเป็น ปุ่ม submit แล้วเรียก js ให้ check ค่า ก่อนส่ง submit ค่ะ

<script language="JavaScript">
function Check() {
var errMsg = '';
var ctrlFocus = null;

if (isEmpty(document.getElementById("txtName").value) || document.getElementById("txtName").value == 'Full Name')
{
errMsg += '- Please Enter Full Name\n';
if (ctrlFocus == null)ctrlFocus = document.getElementById("txtName");
}

if (isEmpty(document.getElementById("txtEmail").value) || document.getElementById("txtEmail").value == 'Email Address')
{
errMsg += '- Please Enter Email Address\n';
if (ctrlFocus == null)ctrlFocus = document.getElementById("txtEmail");
} else {
if (!isValidEmail(document.getElementById("txtEmail").value)) {
errMsg += '- Please Enter a Valid Email Address\n';
if (ctrlFocus == null)ctrlFocus = document.getElementById("txtEmail");
}
}
if (errMsg != '')
{
alert(errMsg);
ctrlFocus.focus();
return false;
}
else
{
document.getElementById('InputJoin').submit();
}

}
</script>


<form name="InputJoin" id="InputJoin" action="join.php" >
<input type="text" name="txtName" id="txtName" value="<?=$name?>" class="FormtxtInput" onfocus="if(this.value=='Full Name'){this.value=''}" />
<input type="text" name="txtEmail" id="txtEmail" value="<?=$email?>" class="FormtxtInput" onfocus="if(this.value=='Email Address'){this.value=''}" />
<input name="mode" id="mode" type="hidden" value="add" />
<!-- <a href="" onclick="javascript:document.InputJoin.submit()"></a>
--> <input type="image" border="0" name="btnSub" id="btnSub" align="top" src="images/btn_join.gif" width="38" height="24" onclick="javascript:Check();" />
</form>