Dynamic image for a Twitter Card for each post / permalink

Does your template contain the <meta> tags which ship with the default templates? See head.html lines 8 through 19

<meta property="og:title" content="{{> title}}">
<meta property="og:description" content="{{> description}}">
<meta property="og:type" content="website">
{{#entry}}
{{#thumbnail.large}}
<meta property="og:image" content="{{{url}}}">
<meta name="twitter:card" content="summary_large_image">
{{/thumbnail.large}}
<meta property="og:url" content="{{{absoluteURL}}}">
{{/entry}}

Yes it does, but it uses a single head template for all pages. So currently I'm using the following in my head, which will generate the Twitter Card using a pre-determined image on sharing: -

    <meta name="twitter:card" content="summary" />
    <meta name='twitter:site' content='@franczv' />
    <meta name="twitter:creator"  content='@franczv' />
    <meta name='twitter:title' content='{{> title}}' /> 
    <meta name="twitter:description"  content='{{> description}} | Link to {{title}}' />
    {{#entry}}
    <meta name="twitter:card" content="summary_large_image">
    <meta name='twitter:image' content='https://blotcdn.com/blog_1fd6e52b11464888b57d1ebdf17eaebb/_image_cache/4057df64-cb97-435b-a4a3-6eedf76a6fae.jpg' />
{{/entry}}

This adds the generic share image I use but I wanted to know what variable would work to use the first image in a post if there was one available.

So what you're really trying to do is add a fallback thumbnail if none exists in the post?

Here's how:

<meta property="og:title" content="{{> title}}">
<meta property="og:description" content="{{> description}}">
<meta property="og:type" content="website">
{{#entry}}
{{#thumbnail.large}}
<meta property="og:image" content="{{{url}}}">
<meta name="twitter:card" content="summary_large_image">
{{/thumbnail.large}}
{{^thumbnail.large}}
<meta property="og:image" content="https://blotcdn.com/blog_1fd6e52b11464888b57d1ebdf17eaebb/_image_cache/4057df64-cb97-435b-a4a3-6eedf76a6fae.jpg">
<meta name="twitter:card" content="summary_large_image">
{{/thumbnail.large}}
<meta property="og:url" content="{{{absoluteURL}}}">
{{/entry}}