Graunt was a good friend of portrait painter John Hales, and there are portraits available of nearly all of Graunt's contemporaries of note, but if he ever had his picture done it must have been lost when his house burned. This photo, by Terry Shapiro, is of Jamie Horton portraying John Graunt in the Denver Center Theatre Company's world premiere production of Anthony Clarvoe's The Living,  3 May 1993.

NOTE: I have learned, from John Moran, of a picture purporting to be that of "Captain John Graunt" located on the internet here. That source says it comes from p.147 of T O'Donnell, History of Life Insurance in its Formative Years, Chicago: American Conservation Company 1936. I have no way of verifying whether this is a genuine portrait of Graunt.