What is image frequency in superheterodyne receivers and how is it rejected?

In superheterodyne receiver with IF frequency fIF, when receiving signal at fRF with LO at fLO = fRF - fIF, an undesired image frequency fimage = fLO + fIF = fRF + 2*fIF also mixes to IF. Image rejection methods: Front-end RF filter (tunable bandpass, difficult at low IF), Image-reject mixer (Hartley or Weaver architecture using phase relationships), and High IF selection (places image further from desired signal, easier to filter but IF processing harder). Image rejection ratio is critical specification for receivers.