After you have finished uploading via your NatureFootage account's Submission Upload Page (please see the How to Upload Footage help article for uploading instructions), you will see your footage start to appear in Cliplog with black thumbnail images and no preview video:

This means that your preview images and video are still processing.  The preview media should appear within a day.  Clips with very long durations (i.e. several minutes long) and a high resolution will take longer to process.  Our team checks new stock footage submissions every day to ensure that preview media is processing correctly and in a timely manner.  If you notice any problems with your preview media, you can let us know by emailing us at: contributors@naturefootage.com