I think this is going to be a bug. I investigated the code, and it seems like the default image control is missing from the term results section. I will make sure to correct that for the upcoming release.
In the meantime, this custom code should help you out. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
(change the $default_image_url variable to the default image URL desired)