I would be looking for a solution that recognises the last item automatically, without having to manually edit a file. If you change the menu around, you may have to edit the atributes - i.e. you add a new last item.
There was no obvious "current" clas to work with that gave the last sibling property (i.e. if null, then this is the last sibling).
It would be possible to perform a recursive earch on all the site map nodes and find each last sibling - then we can clear an image if set.
Unfortunately, dusing the menu databind process, the event we are using does not yet have the full sitemap node list, so the last sibling property is meaningless
If it was easy, everybody would be doing it.