A transparent background on a cell or template will make the use of background colors elsewhere and background images elsewhere easy to incorporate. For example, if you want to use a background image for the entire email template the background color for the template should be set transparent in the STYLE.
Open your email template and navigate to the STYLE tab. Click on the color picker for standard background color and delete the hex code seen at the top of the picker. Repeat for extended background if needed.
You should also consider if any of the cells within the template should be set to transparent. You can change a cell's background color in the Cell Style view when editing an individual cell: