If there is no thumbnail available for your YouTube video, Google returns default, blank image. There are a couple options to fix this issue:
Please note that if YouTube returns multiple blank images, it could be time consuming to fix them manually. In these cases, you can use Operations button in the video list screen to start CSV Importer/Exporter. You can then export your videos to a CSV file, set the thumbnail resolution for the videos using your favorite (Google Spreadsheet is recommended) spreadsheet and import them back using the update option in the import screen.