One should have good respect for this thing called "Imagination"—the Imago is a manifestation of the force of Spirit. To those having proper respect for the word imagination, it can be told that the Magickal Imagination may well be over 50% of the total of Magick itself. Naturally, it is meant that kind of Imagination that is brought to a point of Subjective Reality and most certainly never the non-willed and uncontrolled ramblings of what can hardly be called "mind" in the undisciplined person.
The imagination is one of the most powerful tools that a human has. Through the imagination we can see what is not yet existent, and can change one thing into another. We can fly without wings, see with our eyes closed, hear sounds beyond sounds or silence in place of sound. We can test things in our imagination and work out any problems.
Tesla, the great electrical genius, is said to have been able to design a motor in his imagination, set it running, and then come back to it at a later time and check for wear on the bearings and other parts so he could make improvements in the design.
The Magickal Imagination is a major factor in Magick itself—to the point where "Subjective Reality" is equal to "Normal Reality." At some point, Inner can be substituted for Outer, and that which is imaged happens! With new understanding of Quantum Theory, "Subjective Reality" finds new expression in Magickal practice.
Imago is an idealized mental picture that can be recalled in its perfection at any time. While still in the realm of the Imagination, it is a manifestation of the Force of Spirit, taking it beyond ordinary imaginary images. To illustrate, the Imago was the name of the death mask worn in a special ritual on behalf of the deceased in Ancient Rome to establish his immortality. Similarly, I once had a dream about a "Life Mask" to be molded when a person was at their peak, and then worn ritually to enforce that image of youth, strength, and beauty.
The Magickal Imagination is that process at work—an Imago of perfection imposed to change ordinary reality into the ideal.
This is demonstrated well in Sex Magick where it is the careful role-playing of the imagination that allows the partners to exchange one reality with another. In your ritual, you submerge the personality of your partner into the imagined identity of a god or goddess. It isn't illusion, for the more often this is done, the more attributes of the god or goddess will manifest into the old personality.
The skill of the magician is enhanced by the skilled use of correspondences so that the imagined identity is supported by the established attributes associated with the god or goddess. This is why it is important to memorize and know the various appropriate correspondences.