Congrats on your first mod.
Just tried to install this to be able to give you some feedback. It isn't too clear where you should add $thread_starter_img in the navbar template. Not everybody knows where they have to put the code to make it work, and if you put it at the wrong place, you'll get an annoying error when you want to view a topic.
Maybe you could add to your instructions where you have to place the code so the image will show up where you meant it to be?