My UWP application "Shanghai Metro" has an option to allow user save the metro graph to local file system. The image file is located under project directory, "Assets\shanghaimetro-xl.jpg", and will ask the user to choose a location when saving the image.

This is a very common functionality, it's also easy to achieve in UWP applications. The most easy way to do it is to use my library "Edi.UWP.Helpers" http://www.nuget.org/packages/Edi.UWP.Helpers/

This library is also open source on GitHub: https://github.com/EdiWang/UWP-Helpers/

In the BitmapExtensions.cs, there's an extension method called:

public static Task<FileUpdateStatus> SaveToPngImage(this WriteableBitmap bitmap, PickerLocationId location, string fileName);

The parameter is type of WriteableBitmap, so the first step is to convert your image file to WriteableBitmap:

public static async Task<WriteableBitmap> LoadWriteableBitmap(string relativePath)
{
    var storageFile = await Package.Current.InstalledLocation.GetFileAsync(relativePath.Replace('/', '\\'));
    var stream = await storageFile.OpenReadAsync();
    var wb = new WriteableBitmap(1, 1);
    wb.SetSource(stream);
    return wb;
}

Then you can save the image use this method:

var rmbp = await Helper.LoadWriteableBitmap("Assets/shanghaimetro-xl.jpg");
await rmbp.SaveToPngImage(PickerLocationId.PicturesLibrary, "shmetro");

The example code is:

public static async Task<FileUpdateStatus> SaveToPngImage(this WriteableBitmap bitmap, PickerLocationId location, string fileName) 
{ 
    var savePicker = new FileSavePicker 
    { 
        SuggestedStartLocation = location 
    }; 
    savePicker.FileTypeChoices.Add("Png Image", new[] { ".png" }); 
    savePicker.SuggestedFileName = fileName; 
    StorageFile sFile = await savePicker.PickSaveFileAsync(); 
    if (sFile != null) 
    { 
        CachedFileManager.DeferUpdates(sFile); 


        using (var fileStream = await sFile.OpenAsync(FileAccessMode.ReadWrite)) 
        { 
            BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, fileStream); 
            Stream pixelStream = bitmap.PixelBuffer.AsStream(); 
            byte[] pixels = new byte[pixelStream.Length]; 
            await pixelStream.ReadAsync(pixels, 0, pixels.Length); 
            encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, 
                      (uint)bitmap.PixelWidth, 
                      (uint)bitmap.PixelHeight, 
                      96.0, 
                      96.0, 
                      pixels); 
            await encoder.FlushAsync(); 
        } 


        FileUpdateStatus status = await CachedFileManager.CompleteUpdatesAsync(sFile); 
        return status; 
    } 
    return FileUpdateStatus.Failed; 
}

There's few things to metion:

1. I only wrote "png" format support, you can extend this behaviour yourself. 

2. To make sure other thread don't fuck up with your image, use CachedFileManager.DeferUpdates and CachedFileManager.CompleteUpdatesAsync