AEM Assets Dynamic Media: How to Replace an Auto-Generated PDF Thumbnail with a Custom Thumbnail Rendition?

I uploaded a PDF to AEM Assets with Dynamic Media enabled. A thumbnail is automatically generated, but the crop doesn't look good. I uploaded a custom image rendition in the required dimensions, however AEM continues to display the Dynamic Media-generated thumbnail instead of the custom rendition.

How can I configure AEM/Dynamic Media to use my custom thumbnail image for the PDF asset and override the default generated thumbnail?