Your UIElemet Height and Width values will be 0 when you create them using code. If you want the image in a particular resolution, try to assign the same height and width to the UIElement as well.
The code shown below worked well for me.
public static void SaveElementAsJPG(FrameworkElement element, string ImageName)
{
WriteableBitmap wBitmap = new WriteableBitmap(element, null);
using (MemoryStream stream = new MemoryStream())
{
wBitmap.SaveJpeg(stream, (int)element.ActualWidth, (int)element.ActualHeight, 0, 100);
wBitmap = null;
//Use can either save the file to isolated storage or media library.
//Creates file in Isolated Storage.
using (var local = new IsolatedStorageFileStream(ImageName, FileMode.Create, IsolatedStorageFile.GetUserStoreForApplication()))
{
local.Write(stream.GetBuffer(), 0, stream.GetBuffer().Length);
}
//Creates file in Media Library.
var lib = new MediaLibrary();
var picture = lib.SavePicture(ImageName, stream.GetBuffer());
}
}