add checkbox to button

Hi, i have the next code:
PHP Code:

<span id="review-buttons-container" class="a-center">
            <input type="image" src="<?php echo $this->getSkinUrl('images/btn_place_order.gif') ?>" onclick="review.save();" value="<?php echo $this->__('Place Order') ?>" />
        </span>
    </p>
</div>
<script type="text/javascript">
//<![CDATA[
    var review = new Review('<?php echo $this->getUrl('checkout/onepage/saveOrder') ?>', '<?php echo $this->getUrl('checkout/onepage/success') ?>', $('checkout-agreements'));
//]]>
</script>

PHP:

and i add the follow JavaScript code to create checkbox (the idea is that only when you with the checkbox checked, the button work)

i try:
PHP Code:

        <span id="review-buttons-container" class="a-center">
        <form action="" onSubmit="return check(this);">
            <input name="chkbox" value="please make sure i'm checked" type="checkbox" />
            <input type="image" src="<?php echo $this->getSkinUrl('images/btn_place_order.gif') ?>" onclick="review.save();" value="<?php echo $this->__('Place Order') ?>" />
        </form>    
        </span>
    </p>
</div>
<script type="text/javascript">
//<![CDATA[
function check(form){
                if( form.chkbox.checked )
                    return true;
                else{
                    alert("please check the checkbox");
                    return false;
                }
            }
            
            
    var review = new Review('<?php echo $this->getUrl('checkout/onepage/saveOrder') ?>', '<?php echo $this->getUrl('checkout/onepage/success') ?>', $('checkout-agreements'));
//]]>
</script>

PHP:

but its not work for me my javascript knowledge is not so good, please help me to create this checkbox) Thanks.