You can select a custom thumbnail for your entire Media Library by navigating to the Media Library Settings window. Just click the Media Library Options options link for the desired library, then click "Settings."
From the Media Library Settings window, as the Media Library admin, you'll be able to select a new thumbnail for your video. Just click the image in the upper-right of the window.
You'll see a plus (+) button, which you can click to upload a custom thumbnail from your device.
Click the plus (+) button to select a thumbnail from your device.
Your new thumbnail will take the place of the old thumbnail, if one was already selected. Click "Change" to save.
Now you'll see the new thumbnail on the Media Library settings page. The Media Library thumbnail will also be used if you export the library as a podcast RSS feed.
See also: Visibility and Edit Media Library Settings.