I recommend that you read a bit about diodes. Both silicon diodes in general, and also the schottky variant.

Google will produce a lot of nice links.

A diode is intended to lead current in one direction - not it the reverse direction.

The VCC is expected to be low-impedance - the voltage regulator is expected to strong enough to hold VCC at the voltage the regulator is specified for.

The processor inputs are expected to be high-impedance, to not load external electronics.

That means that an electrostatic discharge to VCC will basically just be shorted.

Since the input pins are high-impedance, they are much easier to destroy - an electrostatic discharge to an input will be able to rise the voltage of the pin.

So you want the diode oriented so that if the input pin gets a higher voltage than VCC, the diode will short down to VCC. That means that you must orient the diode to be "short-circuit" when the input has a higher voltage than VCC, and the diode is non-conductive when the input pin has a lower potential than VCC.

The diode makes the impedance of the input adaptive. For voltages below VCC+Vf, the input impedance will be high - little load on external electronics. When the voltage gets above VCC+Vf, the diode will clamp, making the impedance very low, shorting excess energy to VCC.

If you turn the diode the wrong way, then the diode will be conductive during normal operation and force the input pin always high. With a diode with Vf of 0.4V in the diode and 5V on VCC, an incorrectly oriented diode would result in 5V-0.4V = 4.6V on the input.

Having a diode between input and GND would only be meaningful if you believe that the input can get a voltage lower than 0V. And in that case, the diode would have to be oriented so that it will be conductive when GND is the positive side, and your input is the negative side.