方法
<?php
$size = GetImageSize("img/flag.jpg");
?>
<img src="img/flag.jpg" <?php echo $size[3]; ?> />$size[3] の中身
width="120" height="40"つまり
<img src="img/flag.jpg" width="120" height="40" /><?php
$size = GetImageSize("img/flag.jpg");
?>
<img src="img/flag.jpg" <?php echo $size[3]; ?> /><img src="img/flag.jpg" width="120" height="40" />