There you go:
<Mapping ArtAssetID="mail_base_home" FileName="..\Graphics\firefern_mail_base_home.tga" "></Mapping>
As a hint for others issues you may have:
once you get the name of the pic (browsing them) search for it in the skindefinition.xlm file it will give you the correct ArtAssetID.
(edited

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