The default thumbnail image. The default thumbnail for a video – or a resource that refers to a video, such as a playlist item or search result – is 120px wide and 90px tall. The default thumbnail for a channel is 88px wide and 88px tall.
A high resolution version of the thumbnail image. For a video (or a resource that refers to a video), this image is 480px wide and 360px tall. For a channel, this image is 800px wide and 800px tall.
A higher resolution version of the thumbnail image. For a video (or a resource that refers to a video), this image is 320px wide and 180px tall. For a channel, this image is 240px wide and 240px tall.