Peter, of course an icon of a given saint should have some resemblance to him or her, if there exists a historical description, verbal, or painted portrait. This is not always the case, however. If a saint's actual physical appearance is not known, this does NOT give licence to the iconographer to paint that saint using a living person as a model, as did Michelangelo and other artists, in the manner I mentioned in my last post. Self-portraits in the guise of portraying saints is a particularly arrogant gesture, even in a non-Orthodox artwork.
For many saints, particularly female saints, there exists no record of what they looked like, so an iconographer draws from the life of the saint, the liturgical material written for him or her, etc, and paints an icon consistent with the qualities of that saint: layman/monastic/cleric (of appropriate rank), humble or noble birth, station in life (warrior, unmercenary healer, etc) where this has a spiritual meaning, whether the saint was young or aged at repose, whether he or she died a martyr's death, the list goes on.
It was common practice for many early bishops and other holy ones of renown to have a detailed physical description written of them soon after their death, in part to aid in their iconographic portrayal should they be later glorified as saints. This is why icons of many of the early saints and apostles are so consistent in their portrayal over so many centuries. St John Chrysostom, for instance, is always shown with short, slightly receding dark hair, and a short, neatly-trimmed beard. St Mary of Egypt is shown with unruly white hair, wizened skin (from her many years of ascetic life in the desert), and wearing a rough cloak over one shoulder. More recent saints may have the added advantage of photographic evidence, though the iconographer must be careful to not paint the icon in a photo-realistic style devoid of otherwordliness, nor should there be such features suggesting physical deformity or imperfection (such as wearing spectacles - a common modern mistake), as an icon should show the saint in his or her spiritually perfected, heavenly state, as you rightly stated.