Вы можете попробовать это:
WriteableBitmap resizedImage = new WriteableBitmap(imageToResize);//imageToResize is BitmapImage
using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
using (System.IO.IsolatedStorage.IsolatedStorageFileStream isfs = new IsolatedStorageFileStream(fileName, FileMode.Create, isf))
{
double maxHeight = newWidth;
double maxWidth = newHeight;
double scaleX = 1;
double scaleY = 1;
if (pixHt > maxHeight)
scaleY = maxHeight / pixHt;
if (pixWt > maxWidth)
scaleX = maxWidth / pixWt;</p>
<pre><code> double scale = Math.Min(scaleY, scaleX);
int newWidth1 = Convert.ToInt32(pixWt * scale);
int newHeight1 = Convert.ToInt32(pixHt * scale);
resizedImage.SaveJpeg(isfs, newWidth1, newHeight1, 0, 70);
isfs.Close();
}
}