On the wall hung a large photograph of Sir Anthony sitting at the piano.
Literal
Wall [on-には], piano [toward-に] facing sitting Sir Anthony's large photograph [subject-が] hanging was.
A dense descriptive sentence: the relative clause ピアノに向かって座っているアンソニー卿 ('Sir Anthony sitting facing the piano') modifies 大きな写真. 卿 is an honorific suffix meaning 'Sir/Lord.' The whole thing feeds into the existential かかっていました — 'was hanging.' Rich scene-painting in formal narrative style.