The banner isn’t available through the Helix API, you’d need to use the deprecated v5 Get Channel endpoints https://dev.twitch.tv/docs/v5/reference/channels#get-channel-by-id which will give you a profile_banner field containing the URL for the banner.
If you have a use case for that data being made available in helix you should suggest it on UserVoice https://twitch.uservoice.com/forums/310213-developers but honestly I don’t think the majority of users ever see those images, or even know they exist, so it may be that Twitch doesn’t intend to support them going forward (that’s just speculation at this point though).