How can I access to the css file where is the header's background image?
background: url("http://xxx.es") no-repeat scroll center top / 1600px auto transparent;
It's not in the style.css
This is an inline CSS from source code, normally it's because there is somewhere in the theme options where you can upload an image for header and the code was written to process that case by inserting an inline CSS directly. If you already found that line of code in header.php then you can replace the $header_image with your own image. After that you can strip off the inline CSS, make a class for the header image and add the properties to the style.css file.
You can use code tag in your post when quoting codes so it's easier for us to read the code and help you further.