I'M AFRAID THIS SCRIPT NO LONGER WORKS! MAYBE http://www.student.tugraz.at/kollmitzer/gap_howto.html OR https://github.com/EmelyanenkoK/GAPDownloader DOES. SEE COMMENTS. Google Art Project fullsize image downloader. Specify the page URL and the tiles are downloaded, stitched and trimmed.

google_art_project.rb

# Google Art Project fullsize image downloader.
# By Henrik Nyh <http://henrik.nyh.se> 2011-02-05 under the MIT license.
# Requires Ruby and ImageMagick.
#
# NOTE:
# I'm afraid this script no longer works! See the Gist comments.
#
# Usage e.g.:
#   ruby google_art_project.rb http://www.googleartproject.com/museums/tate/portrait-of-william-style-of-langley-174
#
# You can specify multiple URLs on the command line, separated by space.
# Or you can specify no URLs on the command line and instead list them at the end of this file, one on each line,
# with "__END__" before the list.
#
# On OS X, it sets "Downloaded from" metadata and reveals in Finder.
#
# Can reportedly run on Windows as well, with Ruby from http://www.ruby-lang.org/en/downloads/
# and ImageMagick from http://www.imagemagick.org/script/binary-releases.php#windows
# Note that you may need to edit the TEMP_DIRECTORY/OUTPUT_DIRECTORY below.

require "open-uri"
require "fileutils"
require "rbconfig"

module Kernel
  def windows?
    Config::CONFIG['host_os'].match(/mswin|windows|mingw/i)
  end
end

class GAPDownloader

  # Set this to "jpg" or "tif".
  # jpg is a lot smaller but destructively compressed.
  OUTPUT_EXTENSION = "jpg"

  if windows?
    # Case-sensitive. Use forward slashes, or double-escape backslashes.
    TEMP_DIRECTORY = "C:/WINDOWS/Temp"
    OUTPUT_DIRECTORY = TEMP_DIRECTORY
  else
    TEMP_DIRECTORY = "/tmp"
    OUTPUT_DIRECTORY = "#{ENV['HOME']}/Downloads"
    FileUtils.mkdir_p OUTPUT_DIRECTORY
  end

  # You can lower this if you get ridiculously high-res images otherwise.
  MAX_ZOOM_ALLOWED = 10

  class RuntimeError < StandardError; end

  def initialize(url)
    ensure_image_magick!
    @url = url
    verify_url!
  end

  def download
    get_image_id
    determine_zoom
    get_tiles
    stitch_tiles
    trim
    set_metadata
    done
  end

private

  def ensure_image_magick!
    if !windows? && `which montage`.empty?
      error "You must have ImageMagick installed. Could not find 'montage' in your PATH."
    end
  end

  def verify_url!
    unless @url.to_s.match(%r{\Ahttp://www\.googleartproject\.com/})
      error "Please specify a Google Art Project URL."
    end
  end

  def get_image_id
    @html = open(@url).read
    # Reportedly the data-thumbnail can change in the middle of a long download session, but
    # the encodedInfospotId will not. So if we key local files by the InfospotId, we can
    # check for them if download fails and we start over. Also makes for more human names.
    # If I run into it myself, I may adapt the code to auto-resolve a changed data-thumbnail.
    @thumb_id = @html[/data-thumbnail="(.+?)"/, 1]
    @perma_id = @html[/data-encodedInfospotId="(.+?)"/, 1]
    unless @thumb_id && @perma_id
      error "Couldn't find an image at this URL, sorry!"
    end
  end

  def determine_zoom
    0.upto(MAX_ZOOM_ALLOWED) do |zoom|
      open(tile_url(0, 0, zoom))
      @max_zoom = zoom
    end
  rescue OpenURI::HTTPError => e
    raise unless e.message == "404 Not Found"
  end

  def get_tiles
    @max_x = 999
    @max_y = 999

    0.upto(@max_y) do |y|
      0.upto(@max_x) do |x|
        url = tile_url(x, y, @max_zoom)
        path = tile_path(x, y)
        if File.exists?(path)
          puts "Skipping #{url} (already downloaded)..."
          next
        end
        begin
          data = open(url)  # Raises at 404.
          puts "Getting #{url}..."
          File.open(path, "wb") { |f| f.print data.read }
        rescue OpenURI::HTTPError => e
          raise unless e.message == "404 Not Found"
          if y.zero?
            # Found max x. Start on next row.
            @max_x = x - 1
            break
          else
            # Found max y. We have all tiles, so bail.
            @max_y = y - 1
            return
          end
        end
      end
    end
  end

  def stitch_tiles
    # `montage` is ImageMagick.
    # We first stitch together the tiles of each row, then stitch all rows.
    # Stitching the full image all at once can get extremely inefficient for large images.
    tiles_wide = @max_x + 1
    tiles_high = @max_y + 1

    puts "Stitching #{tiles_wide} x #{tiles_high} = #{tiles_wide*tiles_high} tiles..."

    0.upto(@max_y) do |y|
      tiles = (0..@max_x).map { |x| tile_path(x, y) }.join(' ')
      `montage #{tiles} -geometry +0+0 -tile #{tiles_wide}x1 #{row_path(y)}`
    end

    tiles = (0..@max_y).map { |y| row_path(y) }.join(' ')
    `montage #{tiles} -geometry +0+0 -tile 1x#{tiles_high} #{full_path}`
  end

  def trim
    # Trim the black blocks that may appear on right and bottom.
    # We first add a black border to ensure no other color is trimmed, as described on
    # http://www.imagemagick.org/Usage/crop/#trim
    `convert #{full_path} -bordercolor black -border 1x1 -trim #{full_path}`
  end

  def set_metadata
    # 300 DPI instead of 72 DPI; more sane for printing.
    `convert #{full_path} -density 300 #{full_path}`
    if !windows? && !`which xattr`.empty?
      # Set "Downloaded from" Finder metadata, like Safari does.
      system('xattr', '-w', 'com.apple.metadata:kMDItemWhereFroms', @url, full_path)
    end
  end

  def done
    puts "Done: #{full_path}"
    # Reveal in Finder if on OS X.
    unless windows?
      `which osascript && osascript -e 'tell app "Finder"' -e 'reveal POSIX file "#{full_path}"' -e 'activate' -e 'end'`
    end
  end


  def error(message)
    raise GAPDownloader::RuntimeError, "#{message} (#{@url})"
  end


  def tile_url(x, y, zoom)
    # The subdomain can seemingly be anything from lh3 to lh6.
    "http://lh5.ggpht.com/#{@thumb_id}=x#{x}-y#{y}-z#{zoom}"
  end

  def tile_path(x, y)
    File.join(TEMP_DIRECTORY, "gap-#{@perma_id}-tile-#{x}-#{y}.jpg")
  end

  def row_path(y)
    File.join(TEMP_DIRECTORY, "gap-#{@perma_id}-row-#{@max_zoom}-#{y}.#{OUTPUT_EXTENSION}")
  end

  def full_path
    File.join(OUTPUT_DIRECTORY, "#{@perma_id}.#{OUTPUT_EXTENSION}")
  end

end

if __FILE__ == $0
  urls = ARGV.any? ? ARGV : (defined?(DATA) ? DATA.read.strip.split("\n")  : [])
  puts "Error: No URLs given!" if urls.empty?

  urls.each do |url|
    begin
      GAPDownloader.new(url).download
    rescue GAPDownloader::RuntimeError => e
      puts "Error: #{e.message}"
    end
  end
end

gigafineart.heroku.com works, but does anybody know how they do it?

I'm kind of make it work, see a demo: https://www.dropbox.com/s/gih93ye4v1y6lsu/the_starry_night.jpg?dl=0

I am currently getting black canvases when downloading, and have seen comments of others getting the same