If I want to set a image as background on a textBox I can use this code in the axml:
Provided that MyImage.jpg is a file in the application's current folder, you could write
myTextBox.Background = new ImageBrush(new BitmapImage(new Uri("MyImage.jpg")));
If it's a Resource File, you would have to use a Resource File Pack URI:
myTextBox.Background =
new ImageBrush(new BitmapImage(new Uri("pack://application:,,,/MyImage.jpg")));