I noticed to my dismay that the most recent video I uploaded didn't have a thumbnail.  After poking around a bit, however, I was able to figure out how to upload one.  I thought I would post the process here in case anyone else runs into the same problem or would like to change an existing thumbnail on ToonTube.

  As best I remember, the sequence went something like this:

(1) Log in if you are not already logged in.

(2) Click on your name where it says Signed in as ---------- (with ---------- being your user name, of course).

(3) Click on the videos link to bring up all of the videos you have submitted to ToonTube.

(4) Click on the edit link under the video whose thumbnail you wish to submit or change.

(5) The foregoing will bring you to a screen where you can upload a new thumbnail.  You can also change the title or description there :)  Be sure to click the buttom (OK or SAVE, I forget which) to save your changes.

I'm not sure if I got the foregoing steps exactly right, but it should be close enough that it won't be to hard to figure out :)