Icons aren't always literal, they can convey a meaning that's deeper
one interprets it because St Christopher came from a race that was considered Barbaric
we know from the story that the saint was a very strong man who worships deities, but he later longed to worship a god that's the strongest
then long story short he met our LORD in the form of a child who is stronger than anything, then he repented
the saint was depicted as dog-headed human, why?
The name Barbarian to describe the uncivilized, is speculated to come from the noises dogs make (the barking dogs make) this can be because
dogs are the creatures closest to man, called man's best friend, dogs aren't foreign, yet foreign at the same time, so perhaps that's why Barbarians really are called Barbarians
the saint also acquired human (Roman) tongue to preached to the civilized and martyred, to make the saint dog-headed might've been used to emphasize how foreign he was and the significance of him acquiring he language used by the civilized, since it did make everyone to contemplate God's majesty
Sancte Christoforus, ora pro nobis