I agree whole-heartedly with pretty much everything written here.
I guess a lot of the process begins with the individual putting up their image for viewing by others.
I think it's helpful to others if that person encourages/asks for constructive or criticial feedback. I don't think it should be taken for granted that a person is seeking feedback every time they post an image and I don't mean critical in the negative sense. You know what I mean.
So... if you want feedback/advice/ a critique... then ask for it
I do, however, think it's good form (etiquette) to seek permission before doing a reprocess on someone else's image. That's sometimes an area where misunderstandings can occur.