It follows from the Metadata section that a flavor is also expected to provide icons to represent the various elemental and virtual item types that may arise in a Structure101g model.
These should be provided as "Java-friendly" image files (png, gif, jpg) with a size of 16x16 pixels.
Icons need to be provided for each and every type that may occur, including all virtual types - there are no in-built defaults!
Some sample images are provided in the Developer Guide download in the "icons" directory.