add missing image

Author: fantasai

css3-text/Overview.src.html

@@ -2165,7 +2165,7 @@ <h3 id="line-decoration"><span class="secno">8.1</span>
       which the underline is propagated. The final line of text is fuchsia,
       but the underline underneath it is still the blue underline from the
       anonymous inline element.</p>
-      <p><img src="images/underline-example.gif" alt="Sample rendering of the above underline example"></p>
+      <p><img src="images/underline-example.png" alt="Sample rendering of the above underline example"></p>
       <p>This diagram shows the boxes involved in the example above. The
       rounded aqua line represents the anonymous inline element wrapping
       the inline contents of the paragraph element, the rounded blue line