Magic missile says:
You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range.
Silent image says:
You create the image of an object, a creature, or some other visible phenomenon....
If one caster creates an illusory creature with silent image what happens when another caster targets that illusory creature with magic missile? Assuming of course that the mm caster is shooting first and asking questions later, ie, rather than using his or her action to investigate the illusion is simply choosing to attack it. Or I suppose the magic missile caster might have failed the investigation check. Either way, the magic missile caster believes the illusion to be real.
My own inclination would be to rule that magic missile would in fact appear to hit the illusory creature, but I can't find much to either support that conclusion, or to support another.