Bernd - thanks for getting back to me. The "dummy image" is set, however it is not configured to be included on Detail pages (as far as I can tell).
I came across a support thread for cs_seo extension which allows you to set a fallback image.
https://github.com/clickstorm/cs_seo/issues/92
I used the following typoscript setup on my Detail page.
tx_csseo {
1 = tx_news_domain_model_news
1.fallback {
og_image = fal_media
}
}
In the cs_seo extension I had to set the TSConfig PID to my Detail page, I also included the Page TSConfig "Extend News Records".
I then noticed that on News Detail pages I had two sets of OG metadata. One being generated by ext:News and the other by cs_seo. I removed the following partial include from my Detail Template
<f:render partial="Detail/Opengraph" arguments="{newsItem: newsItem, settings:settings}" />
I now have one set of OG tags and a fallback image in place.