Vissca Do you mean a specific instance, or a specific object (member of the family)?
You can pick an instance by UID, by instance variable value, or using many other conditions.
If you want to pick specific objects, say only "RottenZombie" sprites, you can use this condition:
System Pick Zombies By Evaluate (Zombies.ObjectTypeName="RottenZombie")
Note that ObjectTypeName expression doesn't work in Construct 2.