MiniMagick

Magick::read("image.jpg") do |f|
  f.write("manipulated.jpg")
end

image = MiniMagick::Image.open("input.jpg")
image.resize "100x100"
image.write  "output.jpg"

image = MiniMagick::Image.open("input.jpg")
image.combine_options do |c|
  c.sample "50%"
  c.rotate "-90>"
end
image.write "output.jpg"

http://www.ruby-doc.org/gems/docs/j/jf--mini_magick-3.1/README_rdoc.html