There should actually be a entities.media property available in the JSON, when there are images attached to the tweet.

This is what works for me:

{% set tweets = craft.twitter.get('statuses/user_timeline') %}

{% for tweet in tweets %}
    <li>
        <p>{{ tweet.text }}</p>

        {% if tweet.entities.media is defined %}
            {% for item in tweet.entities.media if item.type == 'photo' %}
                <img src="{{ item.media_url }}">
            {% endfor %}
        {% endif %}
    </li>
{% endfor %}