...a portrait of one of his favourite models seated on an elegant sofa.

您给的句子实际上并不是一个句子，.a portrait是中心词，of one of his favourite models都起到了限定的作用。seat作为动词是一个及物动词，通常有seat oneself, be seated等词组的使用，所以说，seated on an elegant sofa作后置定语，修饰models。我们可以改成定语从句能看得更加清晰：...a portrait of one of his favourite models who is seated on an elegant sofa.