Now it's working. (I've used a wrong method name and made some syntax typos). Case is that assetic image method returns array and so foreach is needed here actually. More info here:
https://github.com/symfony/AsseticBundle/blob/master/Templating/AsseticHelper.php#L65
@cordoval Thanks for infos. Let me know if you find anything else that needs to be fixed here. Regards.