I use below code to set up button background image using a png file with white image.
ImageBrush brush = new ImageBrush();
brush.ImageSource = new Bitma
http://www.mendzapp.com/archives/196
This article explains how to detect the selected theme on the phone (dark or white) and how to use this to change the background of controls. In your case when using an image, I'd suggest creating a second image and load the correct one depending on the selected theme on the phone.
Hope this helps! :)