It would be great if the ability to set a custom cover for a channel was there. Picking up an image from a video of the channel is not the ideal at all times. The thumbnail of videos can be unique to the video while a channel cover should collectively represent the content of the channel.
This just rolled out to all users. We will have a blog on this soon with more details. On any channel, click on "..." and select "upload custom thumbnail". You can also do this from Edit channel experience.