Until today, I have been able to adjust the padding on my header images by clicking the settings gear icon and manually adjusting the numbers. This icon is no longer available. Since it is a header image, there is no expanded menu available. How can I remove the default padding now?
I took a look into your account and the mailing you're working on uses the 'Blank with editable header' template. This template has a hard-coded header image space which doesn't allow control of the header image margin/padding. If you'll instead change the template to the 'Blank without a header' template then you'll be able to place the header image at the top of the mailing and adjust the margin/padding for that as you see fit. I created a copy of your mailing, called 'emma copy' where I've changed the template so you can see how this changes the settings of the top image.
I hope this helps! Let me know if there's anything else I can assist you with. I'm happy to help.