Float a DIV on top of another DIV

I was recently assigned the job of copying a JS popup our previous web developer made. I've got it very similar yet there's one thing I can't get, for the close button (X) to float over the popup in the top right corner (rather than being sat on top right corner of the popup). I've tried with position: values in the CSS and other attributes I've found around Stack overflow, yet none seem to do the trick.

The CSS:

#popup {
    position:absolute;
    display:hidden;
    top:50%;
    left:50%;
    width:400px;
    height:586px;
    margin-top:-263px;
    margin-left:-200px;
    background-color:#fff;
    z-index:2;
    padding:5px;
}
#overlay-back {
    position : fixed;
    top : 0;
    left : 0;
    width : 100%;
    height : 100%;
    background : #000;
    opacity : 0.7;
    filter : alpha(opacity=60);
    z-index : 1;
    display: none;
}
.close-image {
    display: block;
    float:right;
    cursor: pointer;
    z-index:3
}

The HTML:

<div id="overlay-back"></div>
<div id="popup">
    <img class="close-image" src="images/closebtn.png" /><span><img src="images/load_sign.png" width="400" height="566" /></span>
</div>

Just add position, right and top to your class .close-image

.close-image {
   cursor: pointer;
   display: block;
   float: right;  
   z-index: 3;
   position: absolute; /*newly added*/
   right: 5px; /*newly added*/
   top: 5px;/*newly added*/
}