Change the default image based on certain condition
I would suggest using Rules as follows:

You can either include Conditional Rules for a single Rule, or use two rules:

Event: After user registration

Condition: image field is empty (only want it to run if they don't upload an image)

Condition: sex field is girl

Action: set image field to url location of girl default image

New Rule:

same as above except for the condition of sex changes to boy


  The Rules module allows site administrators to define conditionally
  executed actions based on occurring events (known as reactive or ECA
  rules). It's a replacement with more features for the trigger module
  in core and the successor of the Drupal 5 workflow-ng module.