They are similar. With hypnosis, you get into a relaxed state and that can be achieved by listening to words, guided imagery, and other techniques. With guided imagery, your mind is creating a picture in this hypnotic state which may help in a variety of different conditions. Remember that all hypnosis is actually self-hypnosis so you have control over your situation regardless of what technique you are using.
No. Many practitioners use both theraputic modalities but they are not the same thing.
Similar. They are not the same but i frequently use imagery in patients under hypnosis. Think of hypnosis as a very suggestive state. There is a term "waking hypnosis." so one can be skillful in getting a patient to accept a suggestion without a formal induction of a hypnotic trance. How one makes suggestions to a patient is very important with or without hypnosis.