set_desktop.rb

#!/usr/bin/env ruby

require 'uri'
require 'open-uri'
require 'tmpdir'
require 'open3'

module MacOS
  module Desktop
    BACKGROUND_TEMPLATE = "tell application \"Finder\"\n  set myFile to POSIX file %s as string\n  set desktop picture to file myFile\nend tell"

    def self.background=(image)
     begin
        img = URI(image).read
        image = nil
        if img.content_type =~ /^image\/(.*)$/
          Dir::Tmpname.create(['background', ".#{$1}"], Dir.home) do |tmpname, n, opts|
            mode = File::RDWR | File::CREAT | File::EXCL
            perm = 0600
            if opts
              mode |= opts.delete(:mode) || 0
              opts[:perm] = perm
              perm = nil
            else
              opts = perm
            end
            tmpfile = File.open(tmpname, mode, opts)
            begin
              tmpfile.write img
              image = tmpname
            ensure
              tmpfile.close
            end
          end
        end
      rescue URI::InvalidURIError
      end

      if File.readable?(image)
        image = File.realpath(image)
p image
        osa_command = BACKGROUND_TEMPLATE % [image.inspect]
        _, s = Open3.capture2('/usr/bin/osascript', :stdin_data => osa_command)
        return s.success?
      end

      false
    end
  end
end


if ARGV.size != 1
  $stderr.puts "USAGE: $0 <path/to/image or URL>"
  exit 1
end

MacOS::Desktop.background = ARGV[0]