[Victor D]

You can add this script elsewhere you wish within skin1 dir (in a separate file named PopBox.js).
I think placing it in skin1/customer/ is good idea
Then insert in /customer/main/product.tpl right below the lines

{/capture}
{include file="customer/dialog.tpl" }

<script src="customer/PopBox.js"  type="text/javascript"></script>
<script type="text/javascript">
                            popBoxWaitImage.src = "{$ImagesDir}/spinner40.gif";
                            popBoxRevertImage = "{$ImagesDir}/magminus.gif";
                            popBoxPopImage = "{$ImagesDir}/magplus.gif";
                            </script>

And copy these images in your skin1/images directory

So you complete first and second step