I keep a file on my desktop filled with high-resolution artwork- so at my discretion I can swap out any of the artwork on my primary gallery wall or any of the framed pieces in my home. Thankfully, I have access to a large format printer, so that makes things easier, too. A trip to Kinko's or Blick will satisfy that urge too!
I keep my artwork diverse- I pull from antique architectural prints, medical screens, botanical illustrations, and black and white photos of vernacular architecture. There are many great sources for copyright-free images like Vintage Printables, the Library of Congress, and even any of the images from the Hubble Space Telescope.
Steal, poach, alter, and print to your hearts desire on these images and start a folder for your own public domain art.