How do I output image in the loop for an entry?
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 %}