Hi,
The plugin is requesting the images from WordPress itself, normally the URLs should be returned correctly. I guess in this case perhaps some sort of a filter might be missing during the process.
Anyways, based on the pattern, I think might be able to suggest a custom code that would fix the image URLs. 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!