You can upload a thumbnail by using the following steps:
1) Select the video you want in the Main media window.
2) Click the “Files” blade (vertical blue tab) on the right hand side of the screen.
3) Click the “Upload” button to upload the thumbnail of your choice.
4) Select the file you would like to be your thumbnail.
5) Click “Upload” in the upload media dialogue box.
6) Delete the default thumbnail file that you are replacing (the file listed as JPEG in the “FORMAT” column )
7) When the new thumbnail appears go into Basic Settings and click the default content check box and then click “Save.”