To hand-pick the thumbnail / frame that represents your Youtube video:

• go to your Youtube channel
• click on the video you want to edit to select it
• click Edit Video

• click on the thumbnail you want to use (to the right of your video

or:

• click Custom Thumbnail below the samples they've offered (if your account is eligible for custom thumbnails)
• find the image you want to use
• double-click on image to select it

Then:

• click Save changes (upper right)

10122017