Background images
A canvas must contain one of these background types:
1) No background
No background color or image is used.
2) Colored background
A fixed or gradient color is used.
3) Static/fallback background
This image is used when no dynamic image is available.
4) Dynamic background
A dynamic image is used where available - i.e. for each recipient that has a dynamic image associated with the merge tag.
Background images are positioned as the base layer of an image. Text and images are visible on top of them.
