Backgrounds might seem like they should be a simple effect to achieve in HTML email, but that’s not always the case. Outlook 2007, 2010 and even 2013 will give users who rely on the TD background-image property a lot of trouble. Because of these problems many have turned to Vector Markup Language (VML), part of the Office Open XML standards. VML has mostly been scrapped in favor of SVG, but older email clients still use it.
Setting a background image just with a Visual editor does not update the code. Outlook will not show the correct image unless you manually edit the HTML. You can edit HTML manually with Code editor or Visual editor.
There is a wide range of text editors out there that will do the trick, but we recommend Sublime Text (Mac, Windows, Linux).
You can also edit VML code with the StampReady builder.
You can also edit VML code with Mailchimp builder.
It isn’t possible to edit or export the HTML of an email builder email or template. You can, however, access your designs at any time from the template library to use again and again. So you need to get edit Campaign Monitor template file and then import it as a new template.