The thumbnail view of the library window can show a preview image of documents with a blob property holding the image data. If you want to do so, you need to register your document type and configure the name of the blob property you want the thumbnail preview to be generated from. One option is to use the registerImageDocumentType method of the @coremedia/studio-client.cap-base-models/content/contentTypeContext. You can also use the standard plugin ConfigureDocumentTypes, setting the imageProperty as shown below.

import ConfigureDocumentTypes from "@coremedia/studio-client.main.editor-components/configuration/ConfigureDocumentTypes";

//...
new ConfigureDocumentTypes({
  names: "CMMedia,CMImage",
  imageProperty: "data",
  richTextImageBlobProperty: "data",
})

The configured property applies to exactly the given document types, only. It is not inherited by subtypes.