improve the image file finder.
Now review-compile finds a image file from:
1. <imgdir>/<builder>/<chapid>/<id>.<ext>
2. <imgdir>/<builder>/<chapid>-<id>.<ext>
3. <imgdir>/<builder>/<id>.<ext>
4. <imgdir>/<chapid>/<id>.<ext>
5. <imgdir>/<chapid>-<id>.<ext>
6. <imgdir>/<id>.<ext>