If there are no linked images or attached images , it will show the default image. If you don't want the default image,
replace the link to the default image with whatever you want in the settings page. Just add the url to the image you want to use and save.
The last screenshot shows how I replaced the empty space with the site logo.
Tested on Vanilla 2.2
The new Version is very different in that it is now linked to the discussion it came from and also has a tooltip with a preview of the content and you can save the url to your default image in a settings page.
To install please disable and delete the original Plugin you had installed and empty the .ini files in the cache. Then install as usual by uploading it and enabling it in the dashboard. Then go to the settings page and add the url for the image you want as a default in case no image is available.