First Entry from First Sub Category from Top Category Slug in Craft

firsts

{# Get the category from the slug. This will return one CategoryModel #}
{% set topCat = craft.categories.slug('someSlug').first() %}

{# Get the first subcategory under the top level category using the getChildren() method on the category model.  #}
{% set firstSubCat = topCat.getChildren().first() %}

{# Grab the first entry under that category using the relatedTo function and appending the .first() method to return only one item. #}
{% set firstEntry = craft.entries.relatedTo( firstSubCat ).first()  %}

{# Set your image from your asset field #}
{% set image = firstEntry.assetField.first() %}

{# Party #}