Вызов из шаблона
${imagestream("vehicle.image")}

Класс метода
 class  ImageStreamMethod  extends  TemplateMethodModel {
     public  Object exec(List args)  throws  TemplateModelException {
         if  (args.size() !=  1 ) {
             throw   new  TemplateModelException("Wrong arguments");
        }

        InputStream inputStream = (InputStream) Environment.getCurrentEnvironment().getVariable((String) args.get( 0 ));

         return   new  TemplateScalarModel() {
             public  String getAsString()  throws  TemplateModelException {
                 return  "";
            }
        };
    }