Only in Lovecraft's world would a quaint building like the Arkham Historical Society be such a valuable resource in the fight against eldritch horrors. To produce the finished prop, click through to download the high resolution files and then:
1. Print the front image on cardstock and trim it to 4" X 6".
2. Adjust your printer document size to 4" by 6", flip over your trimmed card, and print the back side.
3. Apply stamp.
4. Run the back side of the card through the printer again to apply the postmark.
Optionally, you can follow that up with a rubdown with fine sandpaper along the corners and edges, then apply a light wash of tea to age the finished postcard.