"Preexisting images" refers to the pictures or mental representations that already exist or were formed before something else. It means having a preconceived idea or preconception about something based on previous experiences or knowledge.
Demand often culls his source material from preexisting images found in the media, such as the interior of the Fukushima Daichi power plant after the 2011 tsunami, or the United States Oval Office. (art.thehighline.org)
Rubinstein writes: «Although she never ceased to rely on preexisting images, Drexler described to [interviews Roberta] Fallon sometimes feeling «very guilty» about building her paintings from «something not out of your head.» (painters-table.com)