How do I pull an image from a child entry of a structure
property.imageBackground is an ElementCriteriaModel, which is to say that it always behaves much like an array of assets (even if you are sure there is only one).

So the short answer is that you need:

style="background-image: url({{ property.imageBackground.first.url }});"


The catch here is that this will fail if there is for some reason no such asset. This can happen even if the field is required, and so you should always check. There are several ways to check. This is straightforward:

{% if property.imageBackground|length %}
  {{ property.imageBackground.first.url }}
{% endif %}


Or you can use a loop:

{% for background in property.imageBackground %}
  {{ background.url }}
{% endfor %}


Or a default (this uses twig's ?: operator, and relies on some global image always being there, but you now only have one image to make certain is always there):

{% set background = property.imageBackground.first
   ?: g_config.defaultBackgroundImage.first
%}
{{ background.url }}