I think image is at the base of a poem. Metaphor is the poem.
Natalie Diaz has written about the use of image in her poetry. In "Building The Emotional Image"
If you want to convey fact, this can only ever be done through a form of distortion. You must distort to transform what is called appearance into image.
The mystery lies in the irrationality by which you make appearance - if it is not irrational, you make illustration.
Rebuild the image each time that you use it in order to make it new. Take it apart.Great art is deeply ordered. Even if within the order there may be enormously instinctive and accidental things, nevertheless they come out of a desire for ordering and for returning fact onto the nervous system in a more violent way.
What is it? What is it not? Here is an example from Diaz: http://www.thethepoetry.com/2013/03/poem-of-the-week-natalie-diaz/ Diaz's comment about irrationality intrigues me. Another person might have said "imagination." Her term suggests a greater leap of some sort against logic. It is a willingness to go out of bounds. A subversion but in a good way.
For an amusing and in-depth consideration of the function of image and metaphor read "The Narrative of the Image: A Correspondence by Charles Simic":
In this examination of the narrative related to these two terms, Simic writes: "I kind of fancy "Image is the crucifixion, metaphor is the ascension," " and "Don't you think that reading most contemporary poets one would have to conclude that they have never been to the movies? I know for a fact that they have never heard a country fiddle or a banjo playing!" He argues for being grounded and for the ability to dance.