Getting username in ImageField upload_to path

I would like to include the username in my upload_to directory path for when a user uploads an image. Here is what I currently have --

#model
class Avatar(models.Model):
    avatar = models.ImageField(upload_to='images/%s' %(USERNAME) )
     user = models.ForeignKey(UserProfile)

#form
class ProfilePictureForm(ModelForm):
    class Meta:
        model = Avatar
        fields = ('avatar',)

How would I get the USERNAME in the model to be able to set the upload_to path?