To change your thumbnails follow these steps:

  1. Find your video in the "All Videos" Tab.
  2. Click the three dots on the bottom right to open the video options,
  3. Click edit
  4. Click the thumbnail, you'll be prompted to select a new file.
  5. Once you've selected a new file wait up to a minute for the page to save your changes.

Thumbnails preferred size is: 1280x720

If your thumbnail isn't saving, make sure it is exactly 16:9, i.e. 1280x720 or 1120X630. It needs to be cropped exactly otherwise the thumbnails generated from the video will take precedence.