Where Did Religious Icons Come From?

I once read somewhere that the 'halo' was inherited by Christianity from ancient worshippers of the Sun.  However, although this seems believable I never bothered to verify its historic credibility -

A religious icon is a representation of a saint, the Blessed Virgin or Christ that may be rendered in paint on wood, ceramic mosaic, precious stones or metals, weavings and tapestries, marble, ivory or other media. Most commonly, icons are defined as paintings on wood panels, displayed and venerated in the Eastern Orthodox tradition. They are...