Alienposting is a term used for posting images of animals, such as household pets, that have been purposefully poorly photoshopped to be green with horns. The most popular examples are Geeble and "Boy why you so gnarp gnap👽". These animals are also sometimes posted under indecipherable text, which is presumed to be some sort of alien language.
Mark: Hey, Bob, have you heard of alienposting?
Bob: No, whats that?
Mark: Its basically taking a picture of an animal and turning it green.
Bob:... That sounds stupid.
Mark: Shut up.