Upload new banner to YouTube video page

A result of the video page redesign, aside from video, your branding and subscribe button are now the most visible elements on the page, catching the eye of anyone who views your videos. Here’s how to change your banner: Sign into your Account and go to your channel page; Click "Branding Options" button; Click "Video […]

A result of the video page redesign, aside from video, your branding and subscribe button are now the most visible elements on the page, catching the eye of anyone who views your videos. Here’s how to change your banner: Sign into your Account and go to your channel page; Click "Branding Options" button; Click "Video Page Branding" button; You'll see an option to upload your "Video page banner," and a new option to upload an additional video page banner (specifically for the redesigned video page); Upload your new banner for redesigned page; Click "Save Changes." Your new banner will now appear on redesigned page above the video. Recommended sizing: Height: 25px | Width, flexible but max. of 170px in width.

[Source]