How to make images be randomally placed in Ruby? Flappy Bird clone in ruby

I am making a version of Flappy Bird in ruby. I can't figure out how to get the tube's/obstacles to appear. I need them to appear at any length, as long as their is a gap of a certain amount of pixels. I am stuck. Here is my code if you have any tips for me? Also having a wierd issue with the velocity where it slows majorily down as it gets close to the bottom or lowers a certain amount of pixels.

require 'gosu'
    include Gosu



    class GameWindow < Window

      def initialize
        super(450,900,fullscreen = false,update_interval = 15)
        self.caption = "Flappy You"
        @background = Image.new("/Users/JackSharkey/Desktop/background.jpg", :tileable => false)
        @foreground = Image.new("/Users/JackSharkey/Desktop/foreground.jpg", :tileable => true)
        @birdpic = Image.new("/Users/JackSharkey/Desktop/FlappyStraight.png", :tileable => true)
        @birdx = 0
        @birdy = 300
        #@birdypos = 0
        @scroll_x = 0
        @life = "true"
        @birdVel = 0
        @pixelTime = 0
        @start_time = 0
        #@gravity = 100 #This Means that it will go at 100 pixels/ per second
        #@jump_speed = 250


        #@birdup = Image.new("")
          end
        def draw
          #@background.draw(0, 0, 0)
          @foreground.draw(-@scroll_x, 0, 0)
          @foreground.draw(-@scroll_x + @foreground.width, 0, 0)


          @birdpic.draw(120, @birdy, 0)
        end

        def button_down(button)
          close if button == KbEscape
          if button == KbSpace
            @start_time = Gosu.milliseconds 
            @birdVel = 50 #you decide

          end
          #if button == KbUp
           # fall
          #end
        end


          def update
            @scroll_x += 3
            if @scroll_x > @foreground.width
              @scroll_x = 0
            end
            check
            move

            #@birdVel += birdVel * (update_interval / 1000)
            #@birdy += @birdypos * (update_interval / 1000)
          end

          def fall
              @birdy += 100
          end

          def check

            if Gosu.milliseconds - @start_time > 1000
              @birdVel = 0 
            end
          end

          def move
            #@birdy -= @birdVel - 1
            @birdVel -= 2
            @birdy = @birdy - 0.5 * @birdVel
             if @birdy >= 900
              puts "Game Over"
          end
          if @birdy <= -1
              puts "Game Over"
          end
            puts @birdy
          end


          def score
          end

          def obstacles
          end






    end


      window = GameWindow.new
      window.show