diff --git a/README.md b/README.md
index a1deece..19b63e1 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,7 @@ or
 
 # Creating image from path
 image = Escpos::Image.new 'path/to/image.png', {
-  processor: "ChunkyPng" # or MiniMagick
+  processor: :chunky_png # or :mini_magick
   # ... other options, see following sections
 }
 
diff --git a/lib/escpos/image.rb b/lib/escpos/image.rb
index 61d8e1e..6cd68d1 100644
--- a/lib/escpos/image.rb
+++ b/lib/escpos/image.rb
@@ -16,9 +16,8 @@ class Image
     def initialize(image_or_path, options = {})
       @options = options
 
-      processor_klass_name = options.fetch(:processor)
-      processor_klass = ImageProcessors.const_get(processor_klass_name)
-      @processor = processor_klass.new image_or_path, options
+      processor_klass = get_processor_klass(image_or_path, options[:processor])
+      @processor = processor_klass.new(image_or_path, options)
 
       @processor.process!
     end
@@ -53,5 +52,36 @@ def to_escpos
       ].join
     end
 
+    private
+
+    def get_processor_klass(image, processor)
+      klass = get_processor_klass_from_image(image)
+      return klass if klass
+
+      return default_processor_klass unless processor
+
+      case processor.to_s.downcase.gsub(/[_-]/, '')
+      when 'minimagick' then ImageProcessors::MiniMagick
+      when 'chunkypng' then ImageProcessors::ChunkyPng
+      else raise("Escpos:Image unknown processor value #{processor.inspect}")
+      end
+    end
+
+    def get_processor_klass_from_image(image)
+      case image
+      when ::MiniMagick::Image then ImageProcessors::MiniMagick
+      when ::ChunkyPNG::Image then ImageProcessors::ChunkyPng
+      end
+    end
+
+    def default_processor_klass
+      if defined?(MiniMagick)
+        ImageProcessors::MiniMagick
+      elsif defined?(ChunkyPNG)
+        ImageProcessors::ChunkyPng
+      else
+        raise('Escpos:Image requires either mini_magick or chunky_png gem.')
+      end
+    end
   end
 end
diff --git a/test/lib/escpos/image_test.rb b/test/lib/escpos/image_test.rb
index 1bf075e..8d3003f 100644
--- a/test/lib/escpos/image_test.rb
+++ b/test/lib/escpos/image_test.rb
@@ -40,4 +40,23 @@ def test_image_conversion
     assert_equal IO.binread(file), @printer.to_escpos
   end
 
+  def test_processor_chunky_png_image
+    image = Escpos::Image.new ChunkyPNG::Image.new(8, 8), grayscale: true,
+                              compose_alpha: true, extent: true
+    assert_equal image.processor.class, Escpos::ImageProcessors::ChunkyPng
+  end
+
+  def test_processor_mini_magick_image
+    image_path = File.join(__dir__, '../../fixtures/tux_mono.png')
+    image = Escpos::Image.new MiniMagick::Image.new(image_path), grayscale: true,
+                              compose_alpha: true, extent: true
+    assert_equal image.processor.class, Escpos::ImageProcessors::MiniMagick
+  end
+
+  def test_processor_default
+    image_path = File.join(__dir__, '../../fixtures/tux_alpha.png')
+    image = Escpos::Image.new image_path, grayscale: true,
+                              compose_alpha: true, extent: true
+    assert_equal image.processor.class, Escpos::ImageProcessors::MiniMagick
+  end
 end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 812c543..6082943 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -5,4 +5,7 @@
 require 'pp'
 
 require 'escpos'
+require 'chunky_png'
+require 'mini_magick'
+
 require File.expand_path('../../lib/escpos/image.rb', __FILE__)