From the web, go into your settings. From there you will see "Offline Channel Image" and the option to add the image. Click on the image that says "click to add offline image", select the file you wish to use, and then the upload will show as successful when there is a preview of your selected file.
Please note, images should be 16:9 aspect ratio, in JPG or PNG, and less than 3mb in size.
Additionally, there is a known issue at this time:
Since the channel webpage is cached, sometimes it can take up to 30-45 seconds for the image to show up on your channel after uploading. This does not mean that your submitted image didn't successfully get uploaded, it just means that it is waiting for the cache to catch up. Thank you!