All image URLs must be crawlable and indexable. Otherwise, we will not be able to display them on the search results page.
So this either means that the image is accessible and blocked for crawlers. Then Google states the following:
When specifying an image as a structured data property, make sure that the image actually belongs to the instance of that type. For example, if you define the image property of schema.org/NewsArticle.image, the marked-up image must directly belong to that news article.
Which means that the image should be part of the HTML of the page to markup.
Question is: Should the image live in the html or is it ok for it to live in a separate folder?