Background of ink file is transparent

Question

I am creating an inking app in JavaScript for Windows RT

I am struggling with saving the image.

This is the line of code that does the saving

this.inkManager.saveAsync(saveStream)

The problem I have is that the resulting file has transparent background which looks wrong in many circumstances. 

As for example when sharing it by email the thumbnails inserted in the email look black.

Is there any way to save ink files with white background?

Thanks!

Claudia