You can try Imagetools for Silverlight library in Codeplex. Here is an example on saving a canvas element to JPEG image :
//Convert UIElement to Image
ei = ImageExtensions.ToImage(myCanvas);
//Save the image
SaveFileDialog saveDlg = new SaveFileDialog();
saveDlg.Filter = "JPEG Files (*.jpeg)|*.jpeg";
saveDlg.DefaultExt = ".jpeg";
if ((bool)saveDlg.ShowDialog())
{
using (Stream fs = saveDlg.OpenFile())
{
ei.WriteToStream(fs);
}
}
Further information on how to use it can be found in the link above, and as I see it, looked very simple.