Quoted By:
The Pokedex.
The registered picture MUST match the official Sugimori artwork. (for example, had to be a male Magikarp, female Karp whiskers don't match the artwork)
This was a much bigger problem back when the registered picture was whatever form you first encountered and couldn't be changed.
In later games, if a female was the first one seen and there are no gender differences, I'll make the male form the default.