It has to be:
PHP:
<LeaderheadArtInfo>
<Type>ART_DEF_LEADER_GUSTAV</Type>
<Button>Art/Leaderheads/GustavAdolph/Gustav.dds</Button>
<NIF>Art/Leaderheads/GustavAdolph/VanDerDonk.nif</NIF>
<KFM>Art/Leaderheads/GustavAdolph/VanDerDonk.kfm</KFM>
<NoShaderNIF>Art/Leaderheads/GustavAdolph/VanDerDonk.nif</NoShaderNIF>
<BackgroundKFM>Art/Leaderheads/GustavAdolph/VanDerDonk_BG.kfm</BackgroundKFM>
</LeaderheadArtInfo>
Don't forget to replace the key-names, and to choose the right folder.
edit: i didn't try it out, but so it is supposed to be.