As time wore along, his absorption in the irregular wall and ceiling of his room increased; for he began to read into the odd angles a mathematical significance which seemed to offer vague clues regarding their pnrpose. Old Keziah, he reflected, might have had excellent reasons for living in a room with peculiar angles; for was it not through certain angles that she claimed to have gone outside the boundaries of the world of space we know?
Many of Lovecraft's stories describe "non-Euclidean geometry" as being a gateway to other realities, and the unusual non-perpendicular angles of Keziah's attic room are mentioned a number of times in Dreams in the Witch House. Physically changing the geometry of the hallway isn't feasible, but it's still possible to play with people's sense of perspective to create a feeling of "wrongness" about a space.
Since people generally expect pictures on a wall to be level and rectangular, a long row of slightly-askew pictures seems like a good way to warp the apparent dimensions of a space. Having the pictures shrink gradually from one end of the hall to the other might help to create the illusion that the hall is longer or shorter than it is, and having them slope gradually up or down across the wall could create the impression that the wall itself isn't quite rectangular.
To start with, I selected a few choice images from the Temple of Dagon. We then used Photoshop to ever-so-slightly skew the images, sloping the top edge of each image at 1.6 degrees from the horizontal, and the bottom edge at 2.1 degrees. The cumulative effect over 36 feet would be an extra foot of elevation and a 33% decrease in picture height.
Since you can't put non-orthogonal pictures in standard rectangular frames, I made frames by gluing cardboard strips around the pictures, painting the cardboard brown and black, and varnishing.
Next: Other props