How to display picture
If you need the background image to be dynamic and controlled by the client then I would recommend using inline CSS in the template to set the image path. The rest of the required CSS can be in the CSS file itself.

e.g.,

&lt;div style="background-image: url('{{ entry.image.first().getUrl('transformName') }}')" class="thing" ... &gt;


Your CSS file could even have a fallback background-url
and in your CSS

.thing { background-size: cover; ... }