It's usually not a good idea to explain a joke, and I'm not the GP so I'm not sure that's what they meant, but "plant" has multiple meanings, and I suspect they are referring to meanings 3 to 5 from this list: https://www.oxfordlearnersdictionaries.com/definition/americ... . Basically, you were asking for some kind of elaborate meaning to a random image downloaded by a random app, and they provided an elaborate conspiracy theory to match your question...