How image frequency affect in mixer?

Quick Answer [Knowledge req.]We want to tune into station A, when we do, our radio receiver will mix this desired Station A frequency (RF) with it's Local Oscillator (LO).

The process of multiplting two time domain signals generates signals at sum and difference frequencies (where the IF is the difference frequency). This implies that any signal whose frequency differs from the LO by the same amount that the RF does, will also impose its self on the same IF. E.G. if RF = LO - IF, then the signal at LO + IF will also downconvert and apear at IF. This signal (in this case LO + IF) is called the image frequency IM. and must be attenuated (perhaps by an image rejection filter) prior to mixing.

Where the example above applies to high-side LO injection (LO > RF) the same is true for low-side LO injection (LO < RF).