Porn is not limited to just images, of course. There is a lot of porn in the form of text.

The location of the boundary between soft and hard porn is an interesting question which probably does not have a single "correct" answer. I tend to think of hard porn as being interested exclusively in clinical detail and specific particulars while having little pretensions to being an art form. Soft porn is less single-minded and, um, less hard-edged.

I don't think that "just naked" vs "having sex" is a deciding factor.