diff --git a/spec/api/oembed_spec.rb b/spec/api/oembed_spec.rb
index fcf52e7..1585b94 100644
--- a/spec/api/oembed_spec.rb
+++ b/spec/api/oembed_spec.rb
@@ -1,11 +1,11 @@
 require 'rails_helper'
 
-describe "oEmbed provider" do
+describe "oEmbed provider", needs_phantomjs_2_bin: true do
 
   let(:asciicast) { create(:asciicast) }
 
   it "responds with status 200 for JSON" do
-    get "/oembed?url=http://localhost:3000/a/#{asciicast.id}&format=json"
+    get "/oembed?url=http://localhost:3000/a/#{asciicast.id}&format=json&maxwidth=500&maxheight=300"
     expect(response.status).to eq(200)
   end
 
diff --git a/spec/features/asciicast_spec.rb b/spec/features/asciicast_spec.rb
index 2ac487d..a6d02b4 100644
--- a/spec/features/asciicast_spec.rb
+++ b/spec/features/asciicast_spec.rb
@@ -15,29 +15,4 @@ feature "Asciicast page", :js => true do
     expect(page).to have_selector('.cinema .play-button')
   end
 
-  def rgb(color)
-    [ChunkyPNG::Color.r(color), ChunkyPNG::Color.g(color), ChunkyPNG::Color.b(color)]
-  end
-
-  scenario 'Requesting PNG', needs_phantomjs_2_bin: true do
-    visit asciicast_path(asciicast, format: :png)
-
-    expect(current_path).to match(%r{/uploads/test/asciicast/image/\d+/\w+\.png$})
-
-    png = ChunkyPNG::Image.from_file("#{Rails.root}/public/#{current_path}")
-
-    # make sure there are black-ish borders
-    expect(rgb(png[1, 1])).to eq([18, 19, 20])
-    expect(rgb(png[png.width - 2, png.height - 2])).to eq([18, 19, 20])
-
-    # check content color (blue background)
-    expect(rgb(png[15, 15])).to eq([0, 175, 255])
-
-    # make sure white SVG play icon is rendered correctly
-    expect(rgb(png[png.width / 2, (png.height / 2) - 10])).to eq([255, 255, 255])
-
-    # make sure PowerlineSymbols are rendered
-    expect(rgb(png[144, 795])).to eq([0, 95, 255])
-  end
-
 end
diff --git a/spec/features/png_spec.rb b/spec/features/png_spec.rb
new file mode 100644
index 0000000..4541305
--- /dev/null
+++ b/spec/features/png_spec.rb
@@ -0,0 +1,13 @@
+require 'rails_helper'
+
+feature "asciicast-as-png", needs_phantomjs_2_bin: true do
+
+  let(:asciicast) { create(:asciicast) }
+
+  scenario "Requesting PNG" do
+    visit asciicast_path(asciicast, format: :png)
+
+    expect(current_path).to match(%r{/uploads/test/asciicast/image/\d+/\w+\.png$})
+  end
+
+end
diff --git a/spec/services/asciicast_image_generator_spec.rb b/spec/services/asciicast_image_generator_spec.rb
new file mode 100644
index 0000000..e0cda84
--- /dev/null
+++ b/spec/services/asciicast_image_generator_spec.rb
@@ -0,0 +1,42 @@
+require 'rails_helper'
+
+describe AsciicastImageGenerator, needs_phantomjs_2_bin: true do
+
+  let(:image_generator) { AsciicastImageGenerator.new(template_renderer) }
+  let(:template_renderer) { ApplicationController.new }
+
+  describe '#generate' do
+    let(:asciicast) { create(:asciicast) }
+
+    def rgb(color)
+      [ChunkyPNG::Color.r(color), ChunkyPNG::Color.g(color), ChunkyPNG::Color.b(color)]
+    end
+
+    before do
+      image_generator.generate(asciicast)
+    end
+
+    it 'generates screenshot of "snapshot frame"' do
+      png = ChunkyPNG::Image.from_file("#{Rails.root}/public/#{asciicast.image_url}")
+
+      # make sure there are black-ish borders
+      expect(rgb(png[1, 1])).to eq([18, 19, 20])
+      expect(rgb(png[png.width - 2, png.height - 2])).to eq([18, 19, 20])
+
+      # check content color (blue background)
+      expect(rgb(png[15, 15])).to eq([0, 175, 255])
+
+      # make sure white SVG play icon is rendered correctly
+      expect(rgb(png[png.width / 2, (png.height / 2) - 10])).to eq([255, 255, 255])
+
+      # make sure PowerlineSymbols are rendered
+      expect(rgb(png[144, 795])).to eq([0, 95, 255])
+    end
+
+    it 'sets image_width and image_height on the asciicast' do
+      expect(asciicast.image_width).to_not be(nil)
+      expect(asciicast.image_height).to_not be(nil)
+    end
+  end
+
+end