Thumbnailer extension creating images in /

I’m using middleman-thumbnailer on a blog. It works fine in server mode, but in build mode it tries to generate the thumbnail images in the root of the filesystem rather than the build directory.

# bundle exec middleman build
      create  /img/blog/2013/10/you-have-no-idea-01-large-640x.png
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/fileutils.rb:245:in `mkdir': Permission denied - /img (Errno::EACCES)

This is with middleman 3.2.0.

Gemfile: https://gist.github.com/johngirvin/7338712

config.rb: https://gist.github.com/johngirvin/7338750

Any ideas?