rely on preview urls if available for gallery

test/unit/sources/pixiv_test.rb

@@ -102,6 +102,10 @@ module Sources
           assert_equal("https://i.pximg.net/img-original/img/2017/11/21/05/12/37/65981735_p0.jpg", @site.image_url)
         end
 
+        should "get the preview size image url" do
+          assert_equal("https://i.pximg.net/c/240x240/img-master/img/2017/11/21/05/12/37/65981735_p0.jpg", @site.preview_url)          
+        end
+
         should "get the page count" do
           assert_equal(1, @site.image_urls.size)
         end

test/unit/sources/tumblr_test.rb

@@ -70,6 +70,10 @@ module Sources
         assert_equal("https://media.tumblr.com/3bbfcbf075ddf969c996641b264086fd/tumblr_os2buiIOt51wsfqepo1_1280.png", @site.image_url)
       end
 
+      should "get the preview url" do
+        assert_equal("https://media.tumblr.com/3bbfcbf075ddf969c996641b264086fd/tumblr_os2buiIOt51wsfqepo1_250.png", @site.preview_url)
+      end
+
       should "get the artist" do
         CurrentUser.user = FactoryBot.create(:user)
         CurrentUser.ip_addr = "127.0.0.1"

test/unit/sources/twitter_test.rb

@@ -78,6 +78,10 @@ module Sources
         assert_equal("https://pbs.twimg.com/media/C1kt72yVEAEGpOv.jpg:orig", @site.image_url)
       end
 
+      should "get the preview url" do
+        assert_equal("https://pbs.twimg.com/media/C1kt72yVEAEGpOv.jpg:small", @site.preview_url)
+      end
+
       should "get the canonical url" do
         assert_equal("https://twitter.com/aranobu/status/817736083567820800", @site.canonical_url)
       end