I have this piece of code:
vehicle = get_object_or_404(Vehicle, stock_number=stock_number)
if request.method == 'POST':
    vehicle_form = VehicleForm(request.POST, instance=vehicle)
    photos = PhotosFormSet(request.POST, request.FILES, instance=vehicle)
    if vehicle_form.is_valid() and photos.is_valid():
        vehicle = vehicle_form.save()
        photos.save()
        request.user.message_set.create(message='The vehicle "%s" was edited
          successfully.' % vehicle.__str__())
        return HttpResponseRedirect("/vehicles/details/%s/" % stock_number)
else:
    vehicle_form = VehicleForm(instance=vehicle)
    photos = PhotosFormSet(instance=vehicle)
return render_to_response('vehicles/vehicles-add-edit.html',  
   {'vehicle_form':vehicle_form, 'photos': photos,}, 
   context_instance=RequestContext(request))
The forms are initialized correctly except the ImageField in PhotosFormSet.