so, seems that the following is the most correct, (at least for me worked well so far)
[code]
location ~ "^(/optionalwebappname)?/ProcessImageServlet.*$" {
set $th '';

if ($request_uri ~ "^(.*)[\\?|&]type=th(&.*)?$") {
set $th 'th_';
}
rewrite ^(.+)$ http://static-dev.myhost.eu/$arg_hotel_id/$th$arg_filename break;
expires 1d;
}
[/code]

Again, my problem was to rewrite some urls with GET vars that were expected to contain utf8 characters.
So, the $arg_filename is passed correctly to the rewrite.