cmq credo ke la modifica sia qui

newImage = oldImage.GetThumbnailImage(

(oldImage.Width*ScaleFactor/100), (oldImage.Height*ScaleFactor/100), null, IntPtr.Zero);
nn so .. prova a togliere il /100
newImage = oldImage.GetThumbnailImage(

(oldImage.Width*ScaleFactor), (oldImage.Height*ScaleFactor), null, IntPtr.Zero);