YouTube thumbnails let viewers see an image created from a snapshot of the video while they’re browsing YouTube. Once you have uploaded your video you can choose a thumbnail from one of the three options that YouTube generates automatically for you or upload your own.
To add or change a video thumbnail do the following:
• Sign in to YouTube Studio.
• Select the Content option.
• Select a video by clicking on its title or its thumbnail.
• Choose an auto-generated thumbnail and select Save OR
• Create a custom video thumbnail
To upload a custom thumbnail you must have a verified account. Make your custom thumbnail image as large as possible since it will be used as the preview image in the embedded YouTube player.
Create your custom thumbnails as follows:
• Use a resolution of 1280×720 (and a minimum width of 640 pixels).
• Use standard image formats such as JPG, GIF, or PNG.
• Be smaller than the 2Mb limit.
• Use a 16:9 aspect ratio – this the most often used for previews in YouTube players.