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).