Please make sure that you have all the requirements for the WordPress media library (e.g. GD Library), we use the native WP function for uploading images so WordPress should generate a thumbnail for each of the registered image sizes media_handle_upload() | Function | WordPress Developer Resources
Please send a link to this listing with broken images, we’ll try to debug this further. I’ve seen the same issue with third-party plugins, e.g. when Jetpack CDN was enabled.