What is a placeholder?
A placeholder is something that marks or temporarily fills a place (often used attributively). It can take the shape of a text, image or even a video. Web and mobile designers often use this when they don’t have relevant design assets ready.
Placeholder images are usually light weight and complement the color scheme of the design they are placed in.
Placeholder is a schematic drawing or a very simple and obvious image used as a placeholder for an image that will be added later. The idea is that readers are likely to concentrate on the meaning of the page, rather than the purely visual appearance of it. Wikipedia has an official policy of using empty image placeholders on image-deficient articles (particularly those about real people), essentially accepting this argument.