rely on preview urls if available for gallery

app/logical/sources/strategies/base.rb

@@ -53,6 +53,16 @@ module Sources
         image_urls.first
       end
 
+      # A smaller representation of the image that's suitable for
+      # displaying previews.
+      def preview_urls
+        image_urls
+      end
+
+      def preview_url
+        preview_urls.first
+      end
+
       # Whatever <tt>url</tt> is, this method should return a link to the HTML
       # page containing the resource. It should not be a binary file. It will
       # eventually be assigned as the source for the post, but it does not

app/logical/sources/strategies/pixiv.rb

@@ -50,6 +50,12 @@ module Sources
         [url]
       end
 
+      def preview_urls
+        image_urls.map do |x|
+          x.sub(%r!pximg\.net/img-original/img!, "pximg.net/c/240x240/img-master/img")
+        end
+      end
+
       def page_url
         if novel_id.present?
           return "https://www.pixiv.net/novel/show.php?id=#{novel_id}&mode=cover"

app/logical/sources/strategies/tumblr.rb

@@ -36,6 +36,12 @@ module Sources::Strategies
         .uniq
     end
 
+    def preview_urls
+      image_urls.map do |x|
+        x.sub(%r!_1280\.(jpg|png|gif|jpeg)\z!, '_250.\1')
+      end
+    end
+
     def page_url
       [url, referer_url].each do |x|
         if x =~ POST

app/logical/sources/strategies/twitter.rb

@@ -36,6 +36,12 @@ module Sources::Strategies
     end
     memoize :image_urls
 
+    def preview_urls
+      image_urls.map do |x|
+        x.sub(%r!\.(jpg|jpeg|png|gif)(?::orig)?\z!i, '.\1:small')
+      end
+    end
+
     def page_url
       [url, referer_url].each do |x|
         if self.class.status_id_from_url(x).present?