DAC Nyquist Zones, Zero Order Hold, and Images

why do the Fout images exist in every Nyquist zone? What is a Nyquist Zone?

why do the Fout images decrease in amplitude? The answer is due to frequency response of the zero-order hold function. In Figure below, a zero order hold function is shown in the time domain and frequency domain. The frequency domain plot shown in Figure  is also the sinc function (sin(x)/x). The sinc function has a decreasing amplitude with frequency due to the denominator x