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

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?