W. Fisher, mentioned in JW's correspondence in 1891, may have been the portrait, genre and historical painter William Fisher (b. 1817 in Cork, died 1895 in London). He exhibited at the Royal Academy from 1840-84.
Another possibility is the American born landscape painter William Mark Fisher (1841-1923) who settled in England in 1872 and was a member of the New English Art Club.
Wood, Christopher, Victorian Painters, 2 vols., 3rd ed., revised, Woodbridge, Suffolk, 1995.