The therapist can now construct a metaphor to deliver (in or out of trance) that incorporates the old story and how it has been transformed into a new story - all using the client's images and descriptive language. The client, in effect, tells you how his changes have been brought about. (This approach has parallels with Erickson's "pseudo-orientation in time', but is more structured.)
Of course, the traditional way of utilizing therapist-generated metaphor to suggest a variety of ways that the client can use to resolve her concerns is also useful. (Notice that I am picky about language and prefer 'concerns' to 'problems'.)
Guided imagery is primarily used with people who have life-challenging diseases, although it can also be used for psychotherapy. (The NLP 'swish' technique and the TimeLine Therapy approach can both be considered to be variants of guided imagery.) There is much evidence that body affects mind, and that mind affects body. Some depressions are biochemically related, as is their reversal or control by pharmaceuticals. Psychosomatic ailments are by definition mind-caused. Guided imagery uses the mind for healing, and with a surprising degree of concomitant physical improvement.
The most effective way to do guided imagery work is with client-generated 'images'. What do they feel, or have an inner sense, will work for them? Is it predators or angels? Mechanical devices like ray guns and pulverizers, or gentle persuasion? Biologically accurate things like enhancement of particular immune system components, or a healing presence? The healing metaphor, the healing agency, needs to fit the client's personal belief system and their unique life story. General imagery can be helpful if it is artfully vague. Yet, even with knowledge of the client's specific imagery, it is important to structure your language in as open-ended a way as is possible. The healing image may be an angel, but let the client fill in her own details about the angel- male or female or ... The healing work, after all, is done within the client's mind.
Is it all in the mind? Yes, if you use words. It is not necessary to belabour the point that words like 'red' and 'happy' are individually interpreted. Women, for example, can typically distinguish and name many more shades of red than men can. Yet, each woman experiences a particular red differently.
The placebo effect is always present when you use words since the words evoke individual belief systems. If you congruently believe, and project that belief, that whatever you are doing with a client will be helpful, then your belief tends to become the client's belief. When a treatment is 'new', it is invariably more effective than when it has been around for a while.
In 'talking' therapies words count, so choose them with care.