Twig / Timber if/else

gistfile1.txt

{% if post.thumbnail %}
	<img src="{{post.thumbnail.src}}" />
{% elseif post.heroimage %}
	<img src="{{post.heroimage.src}}|resize(300, 300)" />
{% else %}
	<img src="/wp-content/themes/mytheme/default-image.jpg" />
{% endif %}

You can read the Twig docs on conditionals:
http://twig.sensiolabs.org/doc/tags/if.html