Add fallback styles for image links

This is required by the lightbox plugin. It does not harm blogs
that do not use the lightbox plugin, but makes image links easier
to click.

References #388

templates/default/style_fallback.css

@@ -3,7 +3,7 @@
    Classes are declared on top of the file, so if you
    want to make changes in your own template, you
    should override the rules on the bottom of your
-   style.css template. 
+   style.css template.
    IT IS NOT ADVISED TO CREATE YOUR OWN style_fallback.css FILE! */
 
 /* Default image classes for left/right/usual layout */
@@ -88,4 +88,8 @@
     text-align: center;
 }
 
+.serendipity_image_link {
+    display: block;
+}
+
 /* END OF style_fallback.css */