What is the pivot point of pitch change on an aircraft?
By the very definition, a free-flying object can only pivot around its CG. 

Using anything else as a pivot (e.g. Aerodynamic Center) involves a combination of momentum and translation (linear movement) of the CG and will quickly get you into mathematical problems once you start combining the forces acting on the object. Make no mistake, an object only rotates around its CG. 

If this is just a mental visualisation exercise, you can 'imagine' the pivot point to be at the center of pressure (i.e. the famous plane suspended by strings at the wings), and as the plane is already flying at a few hundred knots in +x, the little +y acceleration error will not make much of a difference as a mental exercise, but the error is there mathematically.

Later edit:if you still have problems visualising it, place your aircraft in outerspace where there is no air or gravity.Put some big thrusters on the wing and smaller thrusters on the tail. CG (center of mass) is very much forward of the wing. Now fire the thrusters.