Image: in its oldest sense, a visual reference in a text, but by extension, a reference to an impression formed by any of the senses, as in "a pattern of auditory imagery."

E.g., in the opening lines of the "General Prologue" to Chaucer's Canterbury Tales, the narrator establishes that spring has come to England by using a series of natural images associated with the season:

Whan that Aprill, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
Whan Zephirus eek with his sweete breeth
Inspired hath in every holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his halfe cours yronne,
And smale foweles maken melodye,
That slepen al the nyght with open eye-
(So priketh hem Nature in hir corages);
Thanne longen folk to goon on pilgrimages, etc.

        The effect of this image pattern is cumulative, creating a population of abstract beings (April and Zephirus) and animated plants and animals (the tender crops and small birds), so that the coming of spring seems to bring to life the entire landscape.  In this context, the poem implies, the "folk" emerge to go on their pilgrimages as if they were impelled by the natural cycle of irresistible renewal rather than for some abstract theological purpose or for cruder reasons.  Some of the images are visual (the sun's passage through the stars of Aries, the birds' open-eyed sleep), but others are tactile (showers piercing the metaphorical root of the drought and bathing its veins in a strength-giving liquid) or auditory (the birds' song). 

        Beginning interpreters of literature often use "image" carelessly, without considering what sense is referred to, whether the sense-image is literal or metaphorical, and specifically how it acts in context.  Without those considerations, merely identifying an image is not much help to the critic. It is especially important to distinguish between images and symbols.