We current have separate tags for photo-editing (205 questions) and image-manipulation (45 questions); in addition, photo-editing has a synonym image-editing. I was trying to write a wiki excerpt for image-manipulation, but couldn't come up with anything which was significantly different in actual meaning from that of photo-editing. Do we think they're synonymous, or is there a difference between the two?
It's probably also worth reading this slightly inconclusive discussion from 2011: Are photo-editing and post-processing synonymous?, and also noting that we have separate tags for post-processing (400 questions) and image-processing (83 questions).
EDIT: OK, looks like we've got a consensus here which mattdm has summarised pretty well in the first three bullets in his answer. I'll try and work up some draft text for all of post-processing, photo-editing and image-manipulation which reflects that.
EDIT 2: I will get round to that write-up soon. Honestly :-)