Channel art is the large banner above your videos that sets the tone for the whole YouTube channel. YouTube shows it differently on phone, computer and TV, so it recommends uploading a 2560×1440 pixel image — the size the canvas opens at. Build the banner from photos: stills from your videos, the team or the channel's theme — and the banner will sit evenly, without cropping or blur.
Because of the different screens, only the central safe area of about 1546×423 pixels is guaranteed to be visible — keep the channel name and logo inside it and fill the rest of the banner with photos. In the editor you can spread shots across the full width, zoom in on the details you want and choose a background. Download the finished banner as PNG or JPG and upload it in your channel art. All free, no sign-up and no watermarks.