You have probably noticed our default audio file thumbnail showing on your file after you upload an audio only file.

When you are logged in, go to your videos, and you should see three little icons below a video or audio thumbnail.  Clcik on the pencil, and that will put you in an edit screen where you can change the name, or do other file procedures.  Scroll further down that page and you'll see a place where you can browse to an image that you can upload to create a new thumbnail image for that file.