Can't get rid of side padding in General Styles e-mail header

In Commerce e-mail global styles, the left and right padding (header) doesn't completely go away. I'm using a heading image, setting padding to 0 to make the image align with the right and left sides of my e-mail content area.   Sizing to fit this header is very wonky! I've tried 2500px wide, 750px wide, and in-between. All come in small when imported, and I have to drag the image height slider to the max just to have the image fit the space. What am I missing?