prawn：Prawn::Images::PNG.new

#!/usr/bin/env ruby

require "prawn"

p Prawn::Document::PNG.new(File.binread("#{Prawn::BASEDIR}/data/images/dice.png"))

で、

#<Prawn::Images::PNG:0xb7cd11b4 @width=320, @transparency={}, @color_type=6, @img_data="...", @interlace_method=0, @palette="", @filter_method=0, @height=240, @compression_method=0, @bits=8>