I'm having a little issue here in showing images.
So when I'm trying to load images from XAML, I can use a relative uri to the image source like this :
<Image Source="/Assets/image.jpg" />
BitmapImage bitmapImage = new BitmapImage(new Uri("/Assets/image.jpg"));
To access files stored inside the application package, but from code where there is no inferred root authority, you need to specify the ms-appx: scheme :
So in your case it will be something like :
BitmapImage bitmapImage = new BitmapImage(new Uri("ms-appx://[project-name]/Assets/image.jpg"));
Read this documentation for more details : https://msdn.microsoft.com/en-us/library/windows/apps/xaml/hh965322.aspx
Hope it helps.