Representative metaphor applies a complete expression, originally suited to one situation, to another situation because of an overall resemblance. Representative simile explicitly compares two situations through a shared composite quality. Both depend on imagery treated as a whole, but their modes of expression differ.
Ask whether a familiar expression is being transferred intact to a new occasion or whether two scenes are expressly presented as comparands. The former invites analysis as a representative metaphor; the latter as a representative simile.
A composite image is not the same thing as a multiword comparison marker. Classification is based on how the intended meaning is constructed, not on the number of words.