Hi BlueWhale, glad to see that other Dark Shadows fans enjoy the recurring props like the Smith Brothers portrait. Not only is the Smith Brothers portrait seen in several places throughout the series, but also the flat for Roger's office at the cannery is one of the most frequently redressed set designs of the show's run, including the front room of Professor Stokes' apartment.
Hello to you PrisoneroftheNight! I can almost swear that the S.Bro's portrait is used at about the same time for Stokes' place and Blairs' place. I laughed with my wife on this one. They must've run out of portrait paintings since most were probably being used in Collinwood and the Old House. ;)