How to change the Poster Image.

This is a very simple thing to do. The first thing you need to do is be sure your YouTube channel is verified. You can do that here.

Once you have verified your page, follow these instructions. Now, YouTube calls the Poster Image a Thumbnail Image.

To upload a custom thumbnail for an existing video:

1. Go to your channel Video Manager.
2. Locate a video and click the Edit button.
3. Click the Custom thumbnail button and upload a thumbnail.
4. Once the thumbnail is uploaded, don't forget to click the "Save Changes" button.

Now go forth and make your websites great!