The Clean Cosby Poster is a poster originally displayed in John Egbert's bedroom, along with many other posters from movies. Later, after entering The Medium, John notices that his posters have now been defaced with harlequin doodles and suspects the Imps as culprits. Rose later sends him a screen shot of his room showing that they've been there all the time.
John later recreates the Clean Cosby Poster by subtracting the code of a Joker Figurine, and hence the doodles, from a Defaced Cosby Poster. He soon combines the clean poster with his computer to create the Cosbytop Computer.