Apply imageTransform on twitter media feed {% set response = craft.twitter.get('statuses/user_timeline', { tweet_mode:'extended', count: 20 }) %}
            {% if response.success %}
            {% for tweet in response.data %}
            {% if tweet.extended_entities.media is defined %}
            {% for item in tweet.extended_entities.media if item.type == 'photo' %}

            &lt;div&gt;&lt;img src="{{ item.media_url.getUrl('footerThumbs') }}" alt=""/&gt;&lt;/div&gt;
            {% endfor %}
            {% endif %}
            {% endfor %}
            {% endif %}


This throws a Runtime error: Impossible to invoke a method ("getUrl") on a string variable ("http://pbs.twimg.com/media/DlAIWwkUcAAY_3u.jpg").

What other method is possible?

Using the Imager plugin:
{% set externalImage = 'http://pbs.twimg.com/media/DlAIWwkUcAAY_3u.jpg' %}

{% set transformedImages = craft.imager.transformImage(externalImage, [
                { width: 200, height: 200 }])
              %}
         {% set response = craft.twitter.get('statuses/user_timeline', { tweet_mode:'extended', count: 20 }) %}
            {% if response.success %}
            {% for tweet in response.data %}
            {% if tweet.extended_entities.media is defined %}
            {% for item in tweet.extended_entities.media if item.type == 'photo' %}

            &lt;div&gt;&lt;img src="{{ externalImage }}" alt=""/&gt;&lt;/div&gt;
            {% endfor %}
            {% endif %}
            {% endfor %}
            {% endif %}}
Craft's built-in transform functionality doesn't support transforming external URLs. You could use my Imager plugin to do this instead.

EDIT:

Based on your example, you'd use Imager something like this:

{% set response = craft.twitter.get('statuses/user_timeline', { tweet_mode:'extended', count: 20 }) %}
{% if response.success %}
    {% for tweet in response.data %}
        {% if tweet.extended_entities.media is defined %}
            {% for item in tweet.extended_entities.media if item.type == 'photo' %}
                {% set transformedImage = craft.imager.transformImage(item.media_url, { width: 200, height: 200 }) %}
                &lt;div&gt;&lt;img src="{{ transformedImage.url }}" alt=""/&gt;&lt;/div&gt;
            {% endfor %}
        {% endif %}
    {% endfor %}
{% endif %}


So, the essential part is that you need to pick up the url to each item in the loop (item.media_url, if that's what the twitter plugin returns), and do the transform inside the for loop.