Good news. The New York Public Library has just added hi-res versions of tens of thousands of its public domain digital holdings and they invite you to use them however you like. Print and frame them for your walls, work them into homemade party invitations, basically, use and remix them to your heart's content. Here are a few of my favorites:
A vibrant blue cyanotype would be striking to frame for your wall. See dozens more in the Photographs of British Algae: Cyanotype Impressions collection.
Looking for something more decorative? How about a botanical by prolific illustrator Pierre Joseph Redouté. There are hundreds to choose from in the Les liliacées collection.
These charming oddities are part of the R.H. Burnside Collection focused on costumes. Make a sign for your desk with one image on each side and let your co-workers know what kind of a mood you're in.