Get a queryset from a queryset
I assume that your models look something like this

class Contest(Model):
    ... something ...

class Picture(Model):
    user = ForeignKey(User)
    contest = ForeignKey(Contest)
    ... something ...

So, Picture.objects.filter(user=user) gives you pictures by a particular user (don't have to specify _id, filters operate on model objects just fine). And to get contests with pictures by a particular user you can do 

pics_by_user = Picture.objects.filter(user=user)
contests_by_user = Contest.objects.filter(id__in=pics_by_user.values_list('contest', flat=True))

There might be an easier way though
I have a queryset from Picture.objects.filter(user_ID=user). The Picture model has "contest_ID" as a foreign key. 
I'm looking to get a queryset of Contests which have Pictures, so from the queryset I already have, how do I pull a list of Contest objects?