Resize image from StorageFile
If you're still looking for a way how to find out the dimensions of a StorageFile image, there is an easy way to do that. I am using it in uwp as well.

I guess it would be less expensive than creation of a new BitmapImage.

        StorageFile image;
        ImageProperties properties = await image.Properties.GetImagePropertiesAsync();
        int height = (int)properties.Height;
        int width = (int)properties.Width;


As well I have this one method which I am using couple of places: 

        private async Task&lt;WriteableBitmap&gt; StorageFileToWbmp(StorageFile file)
              {
        ImageProperties props = await file.Properties.GetImagePropertiesAsync();
        WriteableBitmap bitmap = new WriteableBitmap((int) props.Width, (int) props.Height);
        await bitmap.SetSourceAsync(await file.OpenReadAsync());
        return bitmap;
              }