How to handle mock / placeholder assets in a pattern library?
Two ideas come to mind. 

First, it would be a lot easier to check if an image exists if you send in null in the include instead of an url to the placeholder image, and then let the cart template know what to do if there isn't an actual image. Something like this:

{% include '_partials/card' with {
  title: 'Something',
  description: 'A bit of text',
  image: null
} %}


And in _partials/card:

{% if image!=null %}
  &lt;img src="{{ image.getUrl({ width: 400, height: 200 }) }}"&gt;
{% else %}
  &lt;img src="http://lorempixel.com/400/200/"&gt;
{% endif %}


Second option is to have a dummy image in your Assets library, with a known id, and pass that to the card template. Something like:

{% include '_partials/card' with {
  title: 'Something',
  description: 'A bit of text',
  image: craft.assets({ id: 99 }).first()
} %}


The logic in the card template would be a little bit simpler that way I guess. But maybe you risk that the image is deleted at some point and then everything fails? Depending on the images in your Assets library, maybe you could just pick one at random? 

{% include '_partials/card' with {
  title: 'Something',
  description: 'A bit of text',
  image: craft.assets({ order: 'RAND()' }).first()
} %}


Might give some weird results, but.. Depends on your project.