The easiest solution is to use the drag and drop editor and select a custom background image. Please note that not all email clients support background images. Here's a summary of the Email Clients that can use background images:
Appreciate you taking the time to reply, tnak you! and from my digging it appears to be a little more complicated than I thought. Your idea of using a background image is a nice work around and I'll give it a go for sure. With everything I want to achieve I always run into a list of unsupported email clients.. Some day email will be a bit more flexible:)