What's your favorite source for public domain stuff? Email firstname.lastname@example.org and let us know — we'll add it to the guide!
Image: "Winter: Cat on a Cushion" by Théophile-Alexandre Steinlen, via Art Institute Chicago.
While it won't technically be copyright infringement if you don't cite something from the public domain, it's always a good idea to let people know where you got something!
Thus, you should still cite public domain things in your papers and where ever else you use it.
Use NoodleTools as you would any other source. =)