You can't pass an absolute path in Windows 8 store apps. Try to load your picture into a stream such as FileRandomAccessStream using FileOpenPicker Create a BitmapImage and set the stream as source for the bitmap image and assign it as image source to your image control.
BitmapImage bitmapImage = new BitmapImage();
if (file != null)
{
FileRandomAccessStream stream = (FileRandomAccessStream)await file.OpenAsync(FileAccessMode.Read);
bitmapImage.SetSource(stream);
}
ImageUri = bitmapImage;