Unfortunately, Googling for DS pics isn't as easy as it would be for some other show.
The exterior shots of the Old House--there are some, including the one regularly seen as a scene transition on the series, as well as the publicity shots with JF standing outside the Old House with snow on the ground. These are in several of the books published by KLS, the Almanac's, etc etc. You could probably also find them on various Dark Shadows sites, whether official or fan sites. That's usually where you can find most DS pics that are worth something--meaning that they are clear and you can see them. Some sites to look at are:
www.collinwood.netwww.darkshadowsonline.comwww.jonathanfrid.comwww.laraparker.comAnd others. Check out the little area below each cousin's post to see if they have any links there. I know someone on here has a wonderfrul David Selby site that might also have some good pictures that you are looking for.
As for Lang and Blair's house--I'm sure those were probably real houses somewhere at some point, but where those pictures were taken or how they were obtained, I've got no clue at all.