Practicing with a group of people also helps. Yes, in a sense it is, but hypnosis is really a broader category that includes verbal suggestion and thoughts without images along with guided imagery. For instance, telling yourself in the altered state that you'll be calm and confident qualifies as self-hypnosis. Seeing yourself that way, in sights, sounds or feelings is imagery. Although hypnosis also uses images, it isn't limited to them.
Meditation is also a broader category.
Imagery is a form of meditation. Meditation is any kind of deliberate focus, usually on just one thing or a very narrow band of things. This clears the mind, slows it down and calms and strengthens the meditator. For most westerners, imagery is a little easier to work with, more absorbing and appealing than the more stringent mindfulness meditation. Everyone's needs are different, but you might want to start out a couple of times a day for about 15 minutes each time for 3 or 4 weeks.