With this simple template modification trending threads without image will have an image by default.
Upload a default image to your server, for example /data/assets/default_image.webp (you can create a new folder in the assets folder to put the image in, like custom then the path will be /data/assets/custom/default_image.webp).
In trending_content_item_thread replace this part of code (line 9):
HTML:
<xf:if is="$content.getCoverImage()">
<a href="{{ $content.getContentUrl() }}"...