Use com.nomagic.magicdraw.uml.ElementIcon utility class for retrieving element icon. It also provides methods for retrieving icons by element metatype, stereotype and etc.

   Element element = ...;
   //get element icon
   ResizableIcon icon = ElementIcon.getIcon(element);
 
   //get UML Package icon
   ResizableIcon packageIcon = ElementIcon.getIconByClassType(com.nomagic.uml2.ext.magicdraw.classes.mdkernel.Package.class);