I was scrolling through the reddit came across this guy talking about the moth that looks like bill. So I searched it and that's not the original image so I think it's supposed to show us that bill to control of the moth or turned into a moth. Actually I just searched what moths represent and in some literature they represent death as well as disguise. I came up with something even more so moths are mentioned in The Great Gatsby and are used to describe his guest but as well his love interest Daisy. He compares them to moths saying that they are attracted the champagne, gossip and chance to be seen like moths are to light. But specifically the moths in the great Gatsby represent his desire to capture Daisy. This might be a stretch but I think Gatsby is representative of Bill and Daisy is Representative of Mabel maybe even both Dipper and Mabel. But I'm more compelled to say Mabel since we have seen her get captured and lured with a world of her dreams by Bill. So I think the moth in the image is supposed to represent Mabel but Bill controlling her body or controlling her as we seen him do to Dipper before.