Refactoring: move image-stuff to new Thumbnail class
re-implement overwrite spec.
re-adding spec for large-image detection
Store image in subdirectories.

Other tests fail. refactoring needed.
Fix regressions.
Merge branch 'feature/subdirs' into develop

* feature/subdirs:
  Fix regressions.
  Store image in subdirectories.
  re-adding spec for large-image detection
  re-implement overwrite spec.
  Refactoring: move image-stuff to new Thumbnail class