Many existing plugins, such as CtagsSideKick, contain resources other than Java files (e.g. icons), and you can look at their build.xml files to see how they include it in the .jar file. Here's an example from build.xml of CtagsSideKick:
<jar jarfile="${install.dir}/${jar.name}">
<fileset dir="${build.dir}"/>
<fileset dir="${src.dir}">
<include name="**/*.xml"/>
<include name="**/*.props"/>
<include name="**/*.html"/>
<include name="**/*.txt"/>
<include name="**/*.png"/>
</fileset>
</jar>

If you want to know how to refer to such resources at run-time, you can also take an example from one of these plugins. The following code is from the Tag class of CtagsSideKick, it loads an icon file from the 'icons/' subdirectory of the CtagsSideKick plugin:
URL url = Tag.class.getClassLoader().getResource("icons/" + iconName);
icon = new ImageIcon(url);

Hope this helps.
Shlomy