imagecreatefromstring undefinded?

Hi,

can someone tell me why this piece of code:


    /**
    * Loads an image from a string (e.g. database)
    * @param string the image
    * @param mime mime type of the image
    * @return boolean
    * @access public
    */
    function loadData ($image,$mime) {
        if ( in_array($mime,$this->types) ) {
            $this->source=imagecreatefromstring($image);
            $this->sourceWidth=imagesx($this->source);
            $this->sourceHeight=imagesy($this->source);
            $this->sourceMime=$mime;
            $this->initThumb();
            return true;
        } else {
            trigger_error('Image MIME type '.$mime.' not supported');
            return false;
        }
    }

produces an error telling me imagecreatefromstring is not defined? Urgent.