On the Q mixer side, things get a little more complex. The RF signal in the path is being modulated with a sine carrier, so it is phase shifted -90 degrees from the similar signal in the I mixer. Because the RF component is higher in frequency than the LO and the UI is lower in frequency than the LO, when both signals arrive at the IF frequency they are phase shifted differently. Thereby, ending up with opposing phases. This effect will eventually result in the image rejection once they have combined the I and Q mixer signals again in the 90 degree hybrid.