Sample plugin (named TestPlugin) contents:
TestPlugin.cbplugin:
    TestPlugin.dll  <-- the plugin
    TestPlugin.zip  <-- its resources
    TestPlugin.png  <-- the highlighted settings icon
    TestPlugin-off.png  <-- the non-highlighted settings icon
These are the "easy" stuff to support, since they share a common base name (the plugin's name). To add support for, e.g. toolbar images, will take a little longer...
EDIT: for those wondering, the problem is not how to define additional files to be installed but rather how to know what files each plugin needs when exporting (saving).