header Image

I’ have successfully setup wordpress on http://www.phifgo.com.

at the begining, I was able to change the background image of the blog header , simply by overwriting the file kubrickheader.jpg on

But being a PHP && WORDPRES NEWBIE, I have done some modification on edit theme menu (admin panel)

Now the look is as u can see on http://www.phifgo.com/
I want to know, how to restore the situation, replacing
the black background with
http://www.phifgo.com//wp-content/themes/default/images//kubrickheader.jpg

Thanks by Phifgo NGUYEN TUAN PHONG