Fix YouTube channel banner?

I usually only post surveys here but now I really have a problem.

My YouTube banner looks very pixelated and it's definitely not the picture. Look at the 2 pictures. Once on Twitch and it looks good. And then just terrible on YouTube.

Here's what you should know.

The problem also occurs on the cell phone. It also shows up when I look at it on another account and it's only on my account.

It's the same on the computer, but only on the account where the banner is on it, so it looks normal with others.

Friends told me that this problem is not showing on their cell phones.

I hope you can help me!

Did you stick to it?

The size should also fit - the banner is generally overstretched.