http://phosphor-escence.blogspot.fr/2010/10/install-rmagick-on-mingw-and-msys.html
install Imagemagick 32bit : http://www.imagemagick.org/script/binary-releases.php#windows
gem install rmagick --platform=ruby -- --with-opt-lib=c:/ImageMagick/6.6.4-Q16/lib --with-opt-include=c:/ImageMagick/6.6.4-Q16/include