Why Do These Image-Files go by No Name other than "OIP"?

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Often freely-shared images will go by the file-name "OIP" (and that's the filename my computer will assign to the image, and ask me 'if I want to replace the previously-downloaded file also named "OIP"'). That made me curious, so I searched and found ... that--if anybody knows--they're not making it easy to find! Do you know what "o.i.p." stands for?