Because Nikki was an optional character in the game, sadly this is pretty much the extent of what the internet has to offer.
These are the few images I managed to find through random searches.
♪ The official character drawing
♪ Right before his performance
♪ During his performance ①
♪ During his performance ②
♪ During his performance ③
♪ A Nikki keychain
These are really old and
extremely somewhat embarrassing, but due to the lack of images above we'll leave them up for Nikki's sake.
♪ This used to be on the index
♪ A quick doodle