Two sites in network, how Have a header for each one?

I have a network under my own domain, wich are claratiscar.cat and claratiscar.cat/es
basically are the same blog but in two languages, of course I'm using the same theme: atahualpa but I cannot get a different header for each one.
in my FTP I just can acces to root domain so there's no way to change by ftp the header, I mean if I add an image it will be shown in all blogs. I don't know how I should acces by ftp to /es. I'm afraid is not possible.

I've tried to use a child theme for each one and change only the image header but I've tried to add the file /images/header and it doesn't work. I assume I have to change something in functions or I dont't know...

So, is there any easy way to have a single header for each site?

I've tried to do it with Atahualpa Easy Header Fix but when I upload the files to the server my web shows an error in the functions file.


Thans for any advice!