Marko,
I think I got mixed up along this thread but now it's clear to me you are the creator of the web service and that the web service is hosted with a third party. It's good to know that changing the response type to base64binary worked.
Being the provider of the web service allows you to change the datatypes of its elements so that it can be consumed by clients without much problems. This will also be the case of some other provider that wishes to publish his own web service. He will build it in a way that people can consume it. If he wants to send a binary file or image he will most probably use base64binary as the datatype. Does this answer your question?
Regards,
Ricardo