Why are capacitors high-pass / low pass frequency elements?
For those two circuits, you are measuring the output voltage at the stub on the right and comparing it to the input voltage coming from the left.  The impedance of the capacitor drops as the frequency of the applied voltage rises, as you state, which means that it lets through higher frequency signals easier than lower frequency ones.  In the first circuit, the capacitor is between the input and output, so high frequency signals will transfer between the input and output better.  In the second circuit, the capacitor is between the output and ground, so high frequency signals will be attenuated rather than reach the output.  You can also think of it as a sort of voltage divider with the capacitor acting as a frequency-dependent resistor.