Lacan was not the first to introduce the Latin concept of "Imago" into psychoanalysis (the first one was Jung). The term, related to the notion of "image" expresses the emotional investment of the subject according to Lacan in the images of other people. Regardless of being a part of each and every subject's world, these images are prototypical and universal (this notion obviously draws from Jung's concept of Archetypes). The images manifest themselves in the psyche of the individual and function as stereotypes which influence the manner in which the subject relates to other around him as they are perceived through the prism of the Imago.
When intialy discussing his notion of the Mirror Stage. Lacan associated three familial complexes (complexes familiaux) with a certain Imago. While Jung saw the Imago as having both a positive and negative effect on the subject, for Lacan the tendency is towards the negative side since they act as deceiving agents which arouse an illusion of unity where there is no unity.
see: Lacanian Terminology