Global Image Variables?
seo.twittercard is an ElementCriteriaModel. To access the actual asset, you need to use seo.twittercard.first (or equivalently: seo.twittercard.first()).

However, you should always check that this exists before referencing it:

{% if seo.twittercard|length %}
  &lt;meta(name='twitter:image:', content='{{ seo.twittercard.first.getUrl() }}')&gt;
{% endif %}


And yes, this is a fine way to store images used globally across the site.