Some of the themes available on the Loom generate thumbnails and header images on the fly. To make this possible each post or page can have an image associated with it. The url for this image is stored in the custom field post-image. To make it easier to access, in premium themes an additional panel is shown in the post and page and edit screens where you can add the url.
Images are best uploaded using the normal WordPress media upload features. You can then copy the url of the image into the post-image field.
Images must be located locally on the domain where your WordPress is installed.