Q:

Is it possible to cite from an unknown source without plagiarising?

I am currently writing a document that needs pictures for visual representations of my topic. I’ve been browsing around on the Internet to find such picture. I found a very good picture that could be suitable for my topic. Unfortunately, I do not know how to cite this picture.
The reason why I say I do not know how to cite this picture is because it is a picture from a textbook (or article) that someone decided to upload on Pinterest. I know this because the picture has a small caption at the bottom that says, Figure 1.2: Blah blah blah.
Is it possible to cite something like this? Should I move on and try to find a substitute? My document will be published in the United States.

A:

Plagiarism is about inaccurately claiming a work as your own. Thus, as long as you make it clear that it's not yours and attribute the source as best you can, you need not fear an accusation of plagiarism.
Whether this is an appropriate source to cite, on the other hand, depends on how you are planning to use it and in what degree of formality.  Since this is unclear from your post, I will suggest a few possibilities along a spectrum:

If you are not formally publishing the image and are not dependent on its content, there is likely to be no problem and you can credit very informally. For example, I recently made a presentation that included an humorous image marked: "[Photo Credit: The Internet]".  Even copyright is unlikely to be an issue as long as you do best-effort attribution and aren't annoying a large corporation or unusually litigious artist.
If you are formally publishing the image, then the publication venue will need to have copyright issues resolved. For this, you either need to track down the author or else give up on using the image.  Here, you can use the standard forms for citing a website. That is likely to not be very helpful for anyone trying to trace the image, given how unstable and dynamic Pinterest is, but will at least follow the appropriate forms.
If you are actually relying on the technical content of the image, you probably need to find another source or generate the content yourself. Since you know nothing about the actual ultimate source, there is no scientific credibility to an anonymous snippet on the internet, and it needs to be treated accordingly: for all you know it could be from a UFOlogy pamphlet or a Chick tract.

