[css3-background] add missing file extensions to some images

Author: dauwhe

css-backgrounds/Overview.src.html

@@ -119,7 +119,7 @@ <h2 id="introduction">Introduction</h2>
 negative, but margins have no influence on the background and border.)
 
 <div class="figure">
-  <p><img src="images/box" alt="Diagram of a typical box, showing the
+  <p><img src="images/box.png" alt="Diagram of a typical box, showing the
   content, padding, border and margin areas">
 
   <p class="caption">The various areas and edges of a typical box.
@@ -510,7 +510,7 @@ <h3 id="the-background-repeat">Tiling Images: the 'background-repeat' property</
 </pre>
 
 <div class="figure">
-  <p><img src="images/bg-repeat" alt="A centered background image, with
+  <p><img src="images/bg-repeat.png" alt="A centered background image, with
   copies repeated up and down the border, padding and content
   areas."><!--<SPAN class="dlink">&nbsp;&nbsp;&nbsp;<A
   name="img-bg-repeat" href="images/longdesc/bg-repeat-desc.html"
@@ -532,7 +532,7 @@ <h3 id="the-background-repeat">Tiling Images: the 'background-repeat' property</
 </pre>
 
   <div class=figure>
-    <p><img src="images/bg-space" alt="Image of an element with a dotted background">
+    <p><img src="images/bg-space.png" alt="Image of an element with a dotted background">
 
     <p class=caption>The effect of 'space': the image of a dot is
     tiled to cover the whole background and the images are equally
@@ -2204,7 +2204,7 @@ <h3 id="corner-overlap">Overlapping Curves</h3>
 1.6em (instead of 2em). See borders B in the figure.
 
 <div class=figure id=reduced-radius>
-  <p><img src="images/corner-large-mix" alt="[image: rectangle with two tiny
+  <p><img src="images/corner-large-mix.png" alt="[image: rectangle with two tiny
   rounded corners and two very large ones, on opposite corners]">
 
   <p class=caption>These rounded corner might be the result of 'width:
@@ -2450,7 +2450,7 @@ <h3 id="the-border-image-slice">Image Slicing: the 'border-image-slice' property
 with no <i>specified size</i> and the <i>border image area</i> as the <i>default object size</i>.
 
 <div class=figure>
-  <p><img src="images/slice" alt="Diagram: two horizontal cuts and two
+  <p><img src="images/slice.png" alt="Diagram: two horizontal cuts and two
   vertical cuts through an image">
 
   <p class=caption>Diagram illustrating the cuts corresponding to the