Well, you just do it:
Collision event:
image_index = #
But, if your image_speed is not set to zero, then the subimage is going to change every step, so you might need to set that in your create event.
As for creating the part that shoots off into the room, you'll need to define a new kind of object for that, which has a sprite that looks like the part you want, and which increases hp and goes away when the player touches it.
There's a drag and drop action, and also code, which creates a new object, and you'd put it into the collision event:
instance_create(x,y, obj_name)
You can set properties for the new object like this:
nnn = instance_create(x,y, obj_name);
nnn.speed = 8;
nnn.direction = random(360);
nnn.friction = random(.1)
nnn.image_index = 0;
You see, by storing the "id" of the created part into the variable "nnn", you can then add values to the object to make it move and stuff.