gistfile1.rb

require 'rubygems'
require 'mechanize'
require 'open-uri'

a = Mechanize.new { |agent|
  agent.user_agent_alias = 'Mac Safari'
}

page = a.get("http://tumblr.com/login")

form = page.forms[1]
form['user[email]']  = "email"
form['user[password]'] = "pass"
a.submit form

base_page = 1399993000
skip_num = 500

(0..500).each do |i|
	page = base_page - i * skip_num
	puts page
	page = a.get("http://www.tumblr.com/tagged/cosplay?before=" + page.to_s)

	page.links.each do |link|
		uri = link.uri.to_s
		if uri.include? ".png" or uri.include? ".jpg" or uri.include? ".jpeg" or uri.include? ".gif"
			begin
				open(uri) {|f|
				   filename = uri.to_s.gsub(/[\/:]+/, "")
				   puts filename
				   File.open(filename, "wb") do |file|
				     file.puts f.read
				   end
				}
			rescue
			end
		end
	end
end