How do I output image in the loop for an entry? I've run into an unexpected situation, where ElementCriteriaModel could not be converted to string.

{% set news = craft.entries.section('news').limit(3) %}
{% for news in news %}
    {% set cover = news.promotionNewsImage %}
    {% if cover|length %}
        {{ cover.getUrl('gtLg') }}
    {% endif %}
{% endfor %}


Any thoughts on that please?
Your image input returns and array so you need to reference the first item, even if you only allowed one to be selected.

{% set news = craft.entries.section('news').limit(3) %}
{% for news in news %}
   {% set cover = news.promotionNewsImage %}
   {% if cover|length %}
       {{ cover[0].getUrl('gtLg') }}
   {% endif %}
{% endfor %}