Post your signage of the "litterbug" that you've seen in the field.
(Please excuse my Latin; I just merged "human" and "insect" with "rubbish", but I believe a full taxonomy is required to make it extinct.)
Found in Tennessee...
(https://live.staticflickr.com/65535/51855701920_fab3f72276_b.jpg) (https://flic.kr/p/2n1j2yd)
...and apparently in St. Thomas (the island nation):
(https://live.staticflickr.com/5/8056192_b369e2eb36_b.jpg) (https://flic.kr/p/HhPS)
st_john_litterbug (https://flic.kr/p/HhPS) by jess barron (https://www.flickr.com/photos/popvulture/), on Flickr
Where did this design come from? A Google search also turns up an instance in Metropolis, Illinois:
(https://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Don%27t_be_a_litter_bug_%285633141727%29.jpg/450px-Don%27t_be_a_litter_bug_%285633141727%29.jpg)
It's not in the MUTCD, so what could have possibly made such a bizarre design appear in so many disparate locations?
I can't find a picture, but I remember when I was a little kid, New York City had signs that said, "Littering is filthy and disgusting so don't do it!" I particularly remember signs like that along the State Road between the Marine Parkway Bridge and the entrance gate for the Breezy Point Co-op.
Here's another variety I've seen in the field. It seems that different municipalities or counties just add their name to the bottom on the sign.
(https://upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Haywood_County_TN_Don-t_be_a_Litterbug.jpg/450px-Haywood_County_TN_Don-t_be_a_Litterbug.jpg)
Sourced (https://commons.wikimedia.org/wiki/File:Haywood_County_TN_Don-t_be_a_Litterbug.jpg) from Wikimedia Commons, photo from Thomas R Machnitzki
I remember a jingle on TV
"Please, please don't be a litterbug
Cause every litter bit hurts."
Versions of the OP and first 2 replies were very prominent on Arkansas secondary roads until probably 25 years ago.