1.) A superheterodyne receiver mixes the wanted frequency (Fw) with the local oscillator frequency (Flo) to produce a difference frequency called the intermediate frequency (Fif).

If the local oscillator is above the wanted frequency then: Flo - Fw = Fif.

However, the image frequency, Fim, will generate the same intermediate frequency.

If the local oscillator is above the wanted frequency then: Fim = Flo + Fif

So the image frequency is separated from the wanted frequency by 2 x Fif.

2.) There are lots of mechanisms that can generate in-band spurious responses. If the band is wider than 2 x Fif then the image is one.

There is also the half-IF response which is where the 2nd harmonic of an interferer mixes with the 2nd harmonic of the local oscillator to produce a response that is separated from the wanted frequency by Fif / 2.

More generally, the fundamental or harmonics of any digital clock or oscillator in the receiver can behave as a local oscillator and mix unwanted signals to Fif.

You can reduce the level of these responses by using an image reject mixer, by using a double balanced mixer that rejects even order harmonic responses, by using a more linear low noise amplifier (LNA), by filtering the LNA harmonics before they are applied to the mixer and by shielding the circuit blocks from each other.