OK. So this is my current take on it. Please correct me if needed.
So "Nude" is used for when the girl is not wanting to be nude - when she is being forced or unwantedly manipulated into removing clothes. So that can be used for nude pics with folded arms or crossed legs, or maybe for when someone else's arm is removing clothes? I'm guessing she doesn't have to be completely naked either, but in the process of having clothes removed.
"Ecchi" is for more intimate-style sexy poses or lewd clothing. Maybe the background of the scene looks like more of bedroom/kitchen or other more intimate setting (not a stage). It is either full nudity, partial nudity, or sexy clothing. The girl is relaxed or into her pose, as she is willingly presenting herself sexy.
"Strip" is for less intimate-style sexy poses or lewd clothing. It looks like she's performing with a stage or pole in background, etc. Actively removing clothing or shaking body for viewers.
So you are saying that any of these poses can be nude, partially nude, or even clothed (with implications that clothes are going to come off), but that it's the implied mood of the girl and implied setting mainly that determine the image category more than just whether there is full nudity, partial/sexy clothing, or clothing removal implied?
It's tricky because some pics are difficult to place as they have elements of multiple categories.