signature can have a background image

User Story

1. As an application developer, I want to add a background image (e.g. a watermark) into the Signature component, so that I can save the user draw with that specific image as an image file.
2. As an application developer, I want to add a form image, so that I can save the user drawing with that form as one image file.


3. As an application developer, I want to add a background image (e.g. an underline) into the Signature component, but I want to save the user signature only without the background image.