You are right, and it is quite interesting. Tsarevitch Nikolai Alexandrovich, son of Alexander II and Maria Alexandrovna. born 20 September 1843 at Tsarskoe Selo, and who died on the 24 of April 1865, at Villa Bermon, Nice, seems to have left almost no visual record.
I have never seen a photograph (and he certainly would have had an engagement photograph with Maria Feodorovna), and I have never even seen a painted portrait of him (which he certainly would have had as Tsesarevitch and as head of regiments, etc.)
I don't even know where he is buried--does anyone?
nick