Permanently Edit the Header Image

I’ve seen a lot of posts about how to add the custom code to the “Custom CSS” section of the Theme options. My code works fine for some things, but what I really need is to add a background image to the header area. I have been successful BUTTTTT, I had to do it through the actual code and the editor so it’s only permanent until WP is updated. Then the theme strips it all out.

Site: http://arcosrestaurante.com
Code I put in:
#header {
border-top:#111 3px solid;
background:#42000d url(images/Arcos-Header_2013.png) no-repeat;
background-position:0px 25px;
}

If someone could help me make this addition so that I don’t have to re-upload everything whenever it’s updated I’d appreciate it.