intellij-sdk-code-samples/code_samples/simple_language_plugin/src/com/simpleplugin/SimpleIcons.java

package com.simpleplugin;

import com.intellij.openapi.util.IconLoader;

import javax.swing.*;

public class SimpleIcons {
  public static final Icon FILE = IconLoader.getIcon("/com/simpleplugin/icons/jar-gray.png");
}