First, click on the box where you want to add your background image. Then on the right side of your email editor, in the toolbox, turn on the ''Background image'' switch.
When you uploaded an image as a background to your email and it doesn't fit the box like in the example below, here is what you need to do.
Click ''Edit'' in the toolbox settings of your background image.
2. Click ''Transform'' in the image gallery.
3. Turn on the ''Lock resolution switch''
4. Change the size into a smaller size by adjusting pixels.
5. Click ''Export''