diff --git a/downloader/downloader.py b/downloader/downloader.py
index 478522d..f905156 100644
--- a/downloader/downloader.py
+++ b/downloader/downloader.py
@@ -9,14 +9,6 @@
 logger = logging.getLogger(__name__)
 
 
-class Error(Exception):
-    pass
-
-
-class UnexpectedImageType(Error):
-    pass
-
-
 def configure_logging():
     root_logger = logging.getLogger()
     handler = logging.StreamHandler()
@@ -46,22 +38,6 @@ def _write_to_file(filepath, content):
     open(filepath, 'wb').write(content)
 
 
-def _download_image_data(image_url):
-    image_handle = urllib2.urlopen(
-        urllib2.Request(
-            image_url,
-            headers={
-                'User-Agent':
-                ('Mozilla/5.0 (Windows NT 10.0; Win64; x64) '
-                 'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 '
-                 'Safari/537.36')
-            }))
-    if image_handle.info().type != 'image/jpeg':
-        raise UnexpectedImageType('Expected image/jpeg, got ' +
-                                  image_handle.info().type)
-    return image_handle.read()
-
-
 def _download_image_urls(url_dict, output_root):
     download_queue = Queue.Queue()
 
@@ -88,7 +64,7 @@ def _download_image_urls(url_dict, output_root):
 
         try:
             _write_to_file(item['destination'],
-                           _download_image_data(item['url']))
+                           url.download_image_data(item['url']))
             download_delay = max(download_delay - 1.0, 2.0)
         except urllib2.HTTPError as e:
             logger.warn('Got error trying to download %s: %s', item['url'], e)
@@ -102,13 +78,8 @@ def _download_image_urls(url_dict, output_root):
         time.sleep(download_delay)
 
 
-def dummy():
-    pass
-
-
 def main(args):
     configure_logging()
-    dummy()
     with open(args.input_file) as input_file:
         url_dict = _parse_input_file(input_file)
     logger.info('Read %d input URLs', len(url_dict))
diff --git a/downloader/url.py b/downloader/url.py
new file mode 100644
index 0000000..ee5cd86
--- /dev/null
+++ b/downloader/url.py
@@ -0,0 +1,22 @@
+import urllib2
+
+_USER_AGENT = (
+    'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) '
+    'Chrome/61.0.3163.100 Safari/537.36')
+
+
+class Error(Exception):
+    pass
+
+
+class UnexpectedImageType(Error):
+    pass
+
+
+def download_image_data(image_url):
+    image_handle = urllib2.urlopen(
+        urllib2.Request(image_url, headers={'User-Agent': _USER_AGENT}))
+    if image_handle.info().type != 'image/jpeg':
+        raise UnexpectedImageType('Expected image/jpeg, got ' +
+                                  image_handle.info().type)
+    return image_handle.read()
diff --git a/tests/test_downloader.py b/tests/test_downloader.py
deleted file mode 100644
index 1a09541..0000000
--- a/tests/test_downloader.py
+++ /dev/null
@@ -1,9 +0,0 @@
-import unittest
-
-from downloader import downloader
-
-
-class DownloaderTest(unittest.TestCase):
-
-    def test_dummy(self):
-        downloader.dummy()
diff --git a/tests/test_url.py b/tests/test_url.py
new file mode 100644
index 0000000..8cad493
--- /dev/null
+++ b/tests/test_url.py
@@ -0,0 +1,43 @@
+import unittest
+import urllib2
+
+import mock
+
+from downloader import url
+
+
+class ImagesTest(unittest.TestCase):
+
+    def setUp(self):
+        mock_urlopen_patch = mock.patch(
+            'downloader.url.urllib2.urlopen', autospec=True)
+        self.addCleanup(mock_urlopen_patch.stop)
+        self.mock_urlopen = mock_urlopen_patch.start()
+
+        mock_request_patch = mock.patch(
+            'downloader.url.urllib2.Request', autospec=True)
+        self.addCleanup(mock_request_patch.stop)
+        self.mock_request = mock_request_patch.start()
+
+    def test_download_succeeds_when_server_is_ok(self):
+        mock_handle = mock.Mock()
+        mock_handle.info.return_value = mock.Mock(type='image/jpeg')
+        mock_handle.read.return_value = 'dummy image data'
+        self.mock_urlopen.return_value = mock_handle
+        self.assertEqual(
+            url.download_image_data('http://mock.com/image.jpg'),
+            'dummy image data')
+
+    def test_download_fails_when_server_returns_403(self):
+        self.mock_urlopen.side_effect = urllib2.HTTPError(
+            url='', code=404, msg='', hdrs=None, fp=None)
+        with self.assertRaises(urllib2.HTTPError):
+            url.download_image_data('http://mock.com/image.jpg')
+
+    def test_download_fails_when_content_type_is_not_jpeg(self):
+        mock_handle = mock.Mock()
+        mock_handle.info.return_value = mock.Mock(type='image/png')
+        mock_handle.read.return_value = 'dummy image data'
+        self.mock_urlopen.return_value = mock_handle
+        with self.assertRaises(url.UnexpectedImageType):
+            url.download_image_data('http://mock.com/image.jpg')