They either have their head in the middle of their body and arms on the top, or the only organic section is the head with the rest of the body being some kind of robotic suit or cybernetic enhancements.
They also seem to be very easygoing and polite even when Zim demands "Superweapons." The Plookesians never said they would give these to him, but they give him some they have laying around just to be nice. They display a happy-go-lucky but mildly sarcastic sense of humor (at least this is true for the two representations of the race we get to see). For example, when Dwicky asks if they use their ship to "fly through the stars", Mooshy informs him that "flying through the stars would be pretty stupid, man!"