FiveFrameSwing
Banned
To make sure I understand you correctly, correcting posture in her case would be staying taller? She does look to be leaning some.
When playing catch, while throwing overhand, you likely find that you need to make a minor adjustment from throw-to-throw. You don't normally respond to those adjustments by aiming, but by telling yourself to throw higher/lower/left/right, and your body adjusts to support that goal. If you look closely, you'll generally see players that are 'playing catch' making posture adjustments between throws ... whether they realize it or not.