Find entries with at least 1 image How can I find entries with at least 1 image?

{% set category = craft.categories.group('storyCategories').slug(catSlug).first() %}

{% set params = {
  section : 'stories',
  order : 'postDate desc',
  relatedTo: category,
  limit : '4'
} %}
{% set stories = craft.entries(params).find() %}


I have this, but am not sure how to only get stories entries when the story has an image.
I've just realised I can do this: 

{% set params = { section : 'stories', order : 'postDate desc', relatedTo: category, limit : '4', storyImage: ':notempty:' } %} 


Where "storyImage" is the field name of my image field