ImageScience is a clean and happy Ruby library that generates thumbnails -- and kicks the living crap out of RMagick. Oh, and it doesn't leak memory like a sieve. :) For more information including build steps, see http://seattlerb.rubyforge.org/
Gemfile gem 'image_science', '~> 1.2.3'
install gem install image_science -v 1.2.3