Use sorl.thumbnail

In your templates you would use

{% thumbnail "http://external.server/img/image.png" "200x200" "scale" as im %}
    <img src="{{im.url}}" width="{{im.width}}" height="{{im.height}}">
{% endthumbnail %}

you can also load from any context varable

{% for image in post.images %}
    {% thumbnail image.url "200x200" "scale" as im %}
        <a href={{image.url}}>
            <img src="{{im.url}}" width="{{im.width}}" height="{{im.height}}">
            ....