So as I got no reply to this I wrote the mod myself
This mod makes it so when you click on the intermediate image the full size image loads in it's place in the same window. And vise versa, when you click on the full size image the intermediate image comes back...if javascript is disabled nothing will happen when you click on the intermediate image.
Add this javascript somewhere. I stuck it in the scripts.js file but you could also put it in your template file in the <head> section, or it's own file if you like (as long as you include it in the template head).
intImage = 2;
function swapImage(first, second) {
switch (intImage) {
case 1:
intImage = 2
document.getElementById("img1").src = first
return(false);
case 2:
intImage = 1
document.getElementById("img1").src = second
return(false);
}
}
The you need to (if you haven't already done so) copy the
// Displays a picture function theme_html_picture()
block from the sample theme to your theme.
Then find these lines:
$pic_html = "<a href=\"javascript:;\" onclick=\"MM_openBrWindow('displayimage.php?pid=$pid&fullsize=1','" . uniqid(rand()) . "','scrollbars=yes,toolbar=yes,status=yes,resizable=yes,width=$winsizeX,height=$winsizeY')\">";
$pic_title = $lang_display_image_php['view_fs'] . "\n==============\n" . $pic_title;
$pic_html .= "<img src=\"" . $picture_url . "\" class=\"image\" border=\"0\" alt=\"{$lang_display_image_php['view_fs']}\" /><br />";
$pic_html .= "</a>\n";
and replace with this:
$picfull_url = get_pic_url($CURRENT_PIC_DATA, 'fullsize');
$pic_title = $CURRENT_PIC_DATA['title'];
$pic_html = "<img id=\"img1\" name=\"img1\" src=\"" . $picture_url . "\"Javascript:;\" onclick=\"swapImage('$picture_url', '$picfull_url')\">";
And your done, tested with FF, IE and Opera.