Sounds like there are some inconsistencies between the media library and the post content on the export site. This can happen a few different ways, where the image exists in the file system, but is not in the database for the media library. This can easily happen on a site that has had a database reset, or database imports.
When that has happened to me, I found it easiest to start with a fresh WP install and rebuild the content. Using Divi’s export/import feature ensures your media library matches your content. You have to work through it page by page, post by post, but if you’re packaging a child theme for sale it is a good idea to have a solid site to export from.
An alternate option if you just need it to get working fast, you could try using a plugin to import external content, which will check all the image urls in the content, and download them to the media library if they do not exist.