An image is a poetic device serving two main functions. First, an image is a description so vivid as to leave the reader with an actual sensory image in their mind. Second, it can be intended to make the reader picture or feel something associated with that which is described; i.e. the image connotes something more than just its own concrete depiction, is a simile or metaphor, or symbolizes something. In many cases, these are combined; images are intended to have some sort of sensory impact on the reader, whether overt or not.
Images are typically thought to create a visual representation, but this is not always the case. While this is the most common usage, skilled writers may use imagery to establish a full description by appealing to any or all of the senses.
Usually an image is a single representation; the system of images throughout a work, or the use of such a system to create a sensory environment, is called imagery. If one image is repeated over and over, that is called a motif, or, if slightly altered and built on throughout the work, an extended metaphor. A writer will typically use imagery or motifs to establish a level of meaning besides the superficial in their work.