If the only thing you are after is a background in the contactlist why do it the hard way trying to make a skin? Why not use a script that allows you to set those easily?
As Basilis already mentioned there is a script available doing just that:
Background Changer
The only possible thing you might not like with that method is the fact the image stretches along with the window, but if that doesn't look so bad or doesn't bother you then go for the easy route (at least give it a try to see if that's what you want)
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