js提交前弹出提示框

<form target="_blank" name="f1" method="post" action="sub2.php">

<img style="cursor:pointer;" onClick="b1_onclick()" src="../images/b1.jpg" width="140" height="74">

</form>



<script LANGUAGE="javascript">
<!--
function b1_onclick(){    
    if(confirm("你确定提交?")){ 
        document.f1.submit();
        return true;
    }else{
        return false;
    }
    }
    //-->
</script>