URL url = ClassLoader.getSystemResource("ressources/logo.png");
Toolkit kit = Toolkit.getDefaultToolkit();
Image img = kit.createImage(url);
setIconImage(img);
This is similar to the previous answer, but I need to add a bit of information.
You can still use a direct path to your image (C:/User/logo.png) BUT imagine you give your program to someone else, he wont have the image in that specific path.
So I recemmend you insert it in your project like so:
(I usualy do a sperate package for any ressources).
so it will become ressources/logo.png and it will work for anybody opening your project.