C#
myImg.Source = new BitmapImage(new Uri(@"component/Images/down.png", UriKind.RelativeOrAbsolute)); 

Python
self.teclogomain.Source = Imaging.BitmapImage(System.Uri("C:\Users\sreddy\AppData\Roaming\pyRevit-Master\extensions\TEC.extension\TEC kit.tab\panel2.panel\uitest.pushbutton\icon.png",System.UriKind.RelativeOrAbsolute))