In my opinion, why don't you add imagepoint3 ?
If you really want to do that, there is a solution :
var dist = distance(Sprite.ImagepointX(1),Sprite.ImagepointY(1),Sprite.ImagepointX(2),Sprite.ImagepointY(2))/2
var angl = angle(Sprite.ImagepointX(1),Sprite.ImagepointY(1),Sprite.ImagepointX(2),Sprite.ImagepointY(2))
Create Steam at
X : Sprite.ImagepointX(1) + dist * cos(angl)
Y : Sprite.ImagepointY(1) + dist * sin(angl)