Hello, and thanks for posting! We don't currently support VML or background images in general in our system. Although using VML CSS in an email environment may force rendering of background images in some older versions of Outlook, there are some troublesome drawbacks that can go hand in hand with using that code. Additionally, we don't support background images through any templates available in the Drag and Drop editor. If you have access to Template Builder through your agency account, you might try coding a template that includes a background image, but otherwise, it's not possible.
If you’re interested, you might check out this article I found that covers some of the challenges that can pop up with using VML in an email environment. Please let me know if you have any more questions!