I am sorry that you are frustrated with editing the templates. We are happy to help get you started in your account. If you haven't yet seen these tutorials, they may be helpful. Also, for the header depending on the template and content the blocks may appear different. Some templates have built in padding around blocks to make sure text does not run to the edge of the email. If you are adding an image though you can use Fit to Block to make the image appear full width.
If you would like for us to take a look at a specific template or block, we are more than happy to.