Hi Luke,
To answer the question - yes - a major tenet of Lacan's was that the ego is imaginary - it is constructed through fantasy. The first stage in its construction is an identification with the mirror image (one reason why photos of ourselves appear odd - this how we really look to others!). Before this, the child is just a bundle of emotions, a screaming "thing" of instincts and demands. Through attaining an ego, it loses its original bounlessness (oceanic boundlessness ring any bells?). Another important element of the mirror stage is the way the child is represented in front of the mirror e.g., "oh what a beautiful baby you are, yes you are, yes you are" sort of craic. Another reason why when a little nervous, going on a date for example, we quickly pop to the bathroom and remind ourselves that we are a cohesive agent, that we have boundaries and that we "look good"!