Firstly,
complement and compliment are not alternative spellings of the same word. Secondly, what would an unmarried Scot know about potential Victorian governesses :lol:
I wouldn't have a background image, IMO the margins are enough. If you use a background image to lend 'authenticity' it begs the question - would all correspondents use the same paper. Probably not, Her Ladyship is unlikely to use the same paper as His Lordship's man-of-affairs.
However if you insist, for that letter I would use low-grade white cut edge paper, whereas for Holmes' reply I might use a sheet of dove-grey deckle edge Italian paper - at $3 a pop
BR