Django foreign key drop down

New to django and python so need a little help with a foreign key drop down. Basically, I have a category model and a image model and I want users to be able to choose which category to put the image in. How do I create a drop down for the category in the image form. Are my views and html correct too. I have had a look online but I can't seem to do it myself. I keep getting errors. Thank you.

Here are my models:

class Images(models.Model):
image = models.ImageField(upload_to='images', blank=False)
img_name = models.CharField(max_length=120, blank=True)
img_date = models.DateTimeField(default=now())
img_user = models.ForeignKey(User)
img_cat_id = models.ForeignKey(Categories)

def __unicode__(self):
    return self.img_name

class Categories(models.Model):
cat_descr = models.CharField(max_length =120, blank=False)

def __unicode__(self):
   return self.cat_descr

VIEWS:

@login_required
def upload_images(request):
context = RequestContext(request)
context_dict={}

if request.method == 'POST':  # render the form, and throw it back.
    # take the form data and process it!

    form = UploadImagesForm(request.POST, request.FILES)

    if form.is_valid():
        print 'form is_valid'


        upload_image = form.save(commit=False)
        upload_image.img_user = request.user

        if 'image' in request.FILES:
            upload_image.image  =request.FILES['image']

        upload_image.save()

        return render(request, 'rmb/upload.html', {'upload_image': form})

    else:
        print form.errors

# Not a HTTP POST, so we render our form using two ModelForm instances.
# These forms will be blank, ready for user input.
else:
    form = UploadImagesForm()

    context_dict = {'upload_image': form}
    all_categories = Categories.objects.order_by('-id')
    context_dict['all_categories'] = all_categories
    print context_dict

    return render_to_response('rmb/upload.html', context_dict, context)

FORMS:

class UploadImagesForm(forms.ModelForm):

#cat_list = ModelChoiceField(queryset=Categories.objects.all())

class Meta:
    model = Images
    fields=('image','img_name')

HTML:

{% block body_block %}
    <form id="upload_form" method="post" action="/rmb/upload/"
            enctype="multipart/form-data">

            {% csrf_token %}
             {{ upload_image.as_table }}
            <input type="submit" name="submit" value="Upload" />
     {% for categories in all_categories %}

         <div> {{ categories.id }} </div>
         {{ categories.cat_descr }}
         <input type="submit" name="submit" value="Upload" />
      {% endfor %}


    </form>

{% endblock %}