I'm trying to do this:

    {% for image in entry.illustration %}
        {% if image | length %}
	        <a href="{{ entry.url }}" class="flag__img">
                <img src="{{ image.getUrl }}" alt="{{ entry.altText }}" class="profile__image">
            </a>
        {% else %}	
            <p>No image.</p>		
        {% endif %}
    {% endfor %}

Without any luck. What would be a good way of telling craft to do something else if entry.illustration does not have an image defined?