Mirror Image

• WWizards of the Coast2018-07-13
  Official Ruling
  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.
• WWizards of the Coast2018-07-13
  Official Ruling
  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.
• WWizards of the Coast2018-07-13
  Official Ruling
  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.
• WWizards of the Coast2018-07-13
  Official Ruling
  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.
• WWizards of the Coast2018-07-13
  Official Ruling
  If the copied creature has in its mana cost, X is considered to be 0.
• WWizards of the Coast2018-07-13
  Official Ruling
  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.

Rulings via Scryfall