Not sure if a bug, but just in case.

Env.: JDK9 b108 + OS X 10.10.5 + retina

Please save run the attached test and icons to the same folder, run the test.
The left icon is low-res. (blue), the right is high-res. (green).

Should the naming convention work when using getResourceAsStream()?

Please note that the following construction will work properly (two green icons, as expected):

  Image img1 = Toolkit.getDefaultToolkit().getImage(getClass().getResource("icon.jpg"));
  ...
  b1.setIcon(new ImageIcon(img1));