First off, go to ‘Account Settings’ then select the ‘Design’ tab. You’ll see the boxes below:
If you select ‘default header’, your newsletter will appear with your subject line at the top, then get straight into the content.
If you select ‘custom header’, you can upload an image which will always appear at the top of your newsletter.
Here are the custom header dimensions you should aim for:
Width: 1080px (to provide retina quality)
Height: Any height you prefer
And that’s it! Easy as pie.