sips is an extremely fast image processing utility that ships with OS X. It's not nearly as robust as ImageMagick but therein lies its beauty.
rsips seeks to expand on sips and make it fun to use in everyday scripting, Rakefiles, static compilers, etc.
gem install rsips
require 'rsips'
image = Rsips::Image.new("photo.jpg")
# => @width = "580", @height = "870"
image.vertical? # => true
Long edge resizing
image.resize 300
# => @width = "200", @height => "300"