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. From ActionScript, use the registerImageDocumentType method of the IEditorContext. You can also use the standard plugin ConfigureDocumentTypes, setting the imageProperty for a given set of document types.

This is how the mapping is registered in the editor plugin of CoreMedia Blueprint:

<editor:ConfigureDocumentTypes names="CMPicture,CMImage"
                               imageProperty="data"/>

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