The values_list method returns a ValuesListQuerySet. This means it has the advantages of a queryset. For example it is lazy, so you only fetch the first 25 elements from the database when you slice it.
To convert it to a list, use list().
all_commenter_ids = PhotoComment.objects.filter(which_photo=which_photo).order_by('-id').values_list('submitted_by', flat=True)[:25]
all_commenter_ids = list(all_commenter_ids)
You might be able to start the queryset from your User model instead of using values_list. You haven’t shown your models, so the following code is a guess:
from django.db.models import Q
commenters = User.objects.filter(Q(id=which_photo.owner_id)|Q(photocomment=which_photo))