just a suggestion to clean up your code: store all those numbers in an array:
$dimensions = array (
468 => array ('width' => 468, 'height' => 60),
234 => array ('width' => 234, 'height' => 60),
12090 => array ('width' => 120, 'height' => 90),
12060 => array ('width' => 120, 'height' => 60),
120240 => array ('width' => 120, 'height' => 240),
125125 => array ('width' => 125, 'height' => 125),
8831 => array ('width' => 88, 'height' => 31),
15038 => array ('width' => 150, 'height' => 38)
);
echo '<img src="' . $Imageurl . '" width="' . $dimensions[$_POST['size']]['width'] . '" height="' . $dimensions[$_POST['size']]['height'] . '">';