This Helper I use to store and resize Images (external or not) – it uses GD so make sure to have it, of course this is just an example, so you must change the structure of paths/URLs to your needs.

class Zend_View_Helper_Images
{
private $_name;
private $_source;
static private $_webpath;
static private $_fspath;

public function __construct() {
if(null == $this->_fspath){
$this->_fspath = str_replace('application','',APPLICATION_PATH)."public/images/tumbs/";
$this->_webpath = "/images/thumbs/";
}
}

public function scale($x = false,$y = false)
{

if(!file_exists($this->_fspath.'x'.$x.'y'.$y.$this->_name)){
$infos = getimagesize($this->_source);

switch($infos['mime']) {
case "image/pjpeg":
case "image/jpeg":
case "image/jpg":
$image=imagecreatefromjpeg($this->_source);
break;
case "image/gif":
$image=imagecreatefromgif($this->_source);
break;
case "image/png":
case "image/x-png":
$image=imagecreatefrompng($this->_source);
break;
}
$newImage = imagecreatetruecolor($x,$y);
imagecopyresampled($newImage,$image,0,0,0,0,$x,$y,$infos[0],$infos[1]);

imagejpeg($newImage,$this->_fspath.'x'.$x.'y'.$y.$this->_name);
imagedestroy($newImage);
}

return $this->_webpath.'x'.$x.'y'.$y.$this->_name;

}

public function images($imageurl)
{
$this->_source = $imageurl;
preg_match('#[^/]+\.[\w]+$#',$imageurl,$match);
$this->_name = $match[0];
return $this;
}
}

in that cas the Implementation looks like:

<img src="<?= $this->images($this->some_image_url)->scale(150,200) ?>" alt=""/>