If the chosen creature is copying something else (for example, if the chosen creature is another Mirror Image), then Mirror Image enters the battlefield as whatever the chosen creature copied.

Any enters-the-battlefield abilities of the copied creature will trigger when Mirror Image enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.

If Mirror Image somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

Mirror Image copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the copied creature has {X} in its mana cost, X is considered to be 0.

If the chosen creature is a token, Mirror Image copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Mirror Image doesn't become a token in this case.