How to add a header to the forums?

[orange peel]

I want to add my own header/banner to my forums. I tried renaming the picture to the same name as the default, then uploading ftp and overwriting the old picture. Didnt change. I tried doing it with a different name. Didnt change. Even when

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http://forum.theclubpenguincheats.com/
thats the site. Does the header have to be the same dimensions? Cuz it isnt