2018-07-13: 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.
2018-07-13: 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.
2018-07-13: 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.
2018-07-13: 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.
2018-07-13: If the copied creature has {X} in its mana cost, X is considered to be 0.
2018-07-13: 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 as 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.