require 'rest_client'
cookies = nil
request = RestClient::Request.new(
    :method => :post,
    :url => 'http://www.yoursite.com',
    :payload => {:username=>'your account name',:password=>'password'}
)
request.execute do |response,request,result|
    cookies = response.cookies #save cookies for further use
end

RestClient.Request.new({
    :method => :post,
    :url => 'http://www.yoursite.com/upload.php',
    :cookies => cookies,
    :payload => {
  :multipart => true,
  :filepath =>[File.new('a.jpg'),File.new('b.jpg')]
    }
}).execute do |resp,req,result|
    $stdout.puts resp
end