drf.py

class MySerializer(serializers.ModelSerializer):
    avatar = serializers.CharField(source='get_avatar')
    avatar_bw = serializers.CharField(source='get_avatar', color='bw')
    
    class Meta:
        model = MyModel
        fields = ('avatar', 'avatar_bw',)
    
    
class MyModel(models.Model):
    avatar = models.ImageField(...)
    
    def get_avatar(self, color='normal'):
        if color == 'bw':
            return 'bw image'
        
        return 'normal image'