There are two scenarios in which a "Non existing uploaded picture" message will appear beneath a '?' placeholder image in your Media-Library.
The first scenario is if an image has been deleted from your account but has a backup existing either on our servers or in your own private S3 bucket. In such a case, an instance of the resource will still be visible in your Media-Library to allow for the restoration of previous versions of the image. On the resource page, you can click the "View backed up versions" button to browse through the backed-up versions of the resource and restore one of them by clicking the respective 'Restore' button.
The second scenario is when requesting an image that doesn't exist on your account, while utilizing our default-image feature. In this case, instead of returning a 404 response, a placeholder image will be generated and stored on your account for all future requests. This placeholder image can later be overridden using the real image, to allow proper access for future requests.