What's going on your site is that you're using the image as a background on the popup. When you popup the window on a mac, it checks out the size you tell it and pops a window that size just like it should.
Windows however, puts scrollbars on everything, but if theres nothing to scoll, it takes them away, but doesn't take away the space for the scrollbars. Am I even making sense? Basically, the windows browser pops up a window bigger than you tell it to. It's stupid.
An idea for a workaround would be instead of making the picture a background image (which is why it's tiling), do something like this:
<table border=0 cellspacing=0 cellpadding=0 width=100% height=100%>
<tr>
<td width=100% height=100% valign=center align=center>
<img src="whaterver.jpg">
</td>
</tr>
</table>
and in the body tag you need:
<body marginwidth=0 marginheight=0 leftmargin=0 rightmargin=0 topmargin=0 bottommargin=0>
That will make it stay in the center even if the thing is a little off.
It's a bunch of crap in my opinion, that stuff should all be default. Some of it is, I guess. Oh well.
That's what I've done to overcome those ob-stackles.
By the way, those pictures rock.