Use png instead of tiff

svn path=/trunk/aspeditor/; revision=60706

src/propertygrid/AspNetEdit.UI/PropertyGrid.cs

@@ -459,7 +459,7 @@ public class PropertyGrid : Gtk.VBox
 	private Gdk.Pixbuf ImageToPixbuf(System.Drawing.Image image)
 	{
 		using (MemoryStream stream = new MemoryStream ()) {
-			image.Save (stream, System.Drawing.Imaging.ImageFormat.Tiff);
+			image.Save (stream, System.Drawing.Imaging.ImageFormat.Png);
 			stream.Position = 0;
 			return new Gdk.Pixbuf (stream);
 		}