You can edit the placeholder URL manually in wp-content/plugins/recent-posts-widget-extended/includes/functions.php
It looks like this (http):
'thumb_default' => 'http://placehold.it/45x45/f0f0f0/ccc',
You can change it to this (https):
'thumb_default' => 'https://placehold.it/45x45/f0f0f0/ccc',
In that case it will not break HTTPS. Bonus tip: you can change the size of the placeholder by editing the dimension. In. eg. “…/45×45/” to “../400×280/”
