How to make an img tag item visible/invisible in display HTML

I am using an img tag in html that I want to not be visible when the page loads (that part works)

But I want to programatically make it show up and am apparently doing something wrong because the code
executes (I have an alert before and after) but the image does not show up...

Perhaps some kind of syntax error?


make invisible

<div style="position:absolute; left: 210px; top: 90px;" >
<img name="addtag" id="addtag" style="display:none";  src="jpg/AddTagGreenUp.jpg"
                                 onclick="return ModalPopUp('AddTag.htm')">
</div>


make visible?
 alert('b4');
      document.getElementById('addtag').display = 'block';
      alert('aft');