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>