I know this post is little bit old but here is a potential solution for anyone who has the same problem:
First, I would change the CSS display for #popup to "none" instead of "hidden".
Second, I would change the HTML as follow:
<div id="overlay-back"></div>
<div id="popup">
<div style="position: relative;">
<img class="close-image" src="images/closebtn.png" />
<span><img src="images/load_sign.png" width="400" height="566" /></span>
</div>
</div>
And for Style as follow:
.close-image
{
display: block;
float: right;
cursor: pointer;
z-index: 3;
position: absolute;
right: 0;
top: 0;
}
I got this idea from this website (kessitek.com). A very good example on how to position elements,:
How to position a div on top of another div
I hope this helps,
Zag,