refactoring find_pathes for images.

 1. ``<basedir>/<chapid>-<id>.<ext>``
 2. ``<basedir>/<builder>/<chapid>-<id>.<ext>``
 3. ``<basedir>/<chapid>/<chapid>-<id>.<ext>``
 4. ``<basedir>/<chapid>/<id>.<ext>``
 5. ``<basedir>/<builder>/<chapid>/<id>.<ext>``
 6. ``<basedir>/<builder>/<id>.<ext>``
 7. ``<basedir>/<id>.<ext>``