add alt text to <img>s in Images section of CSS docs

css.html

@@ -2012,14 +2012,14 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
       <p>Keep in mind that Internet Explorer 8 lacks support for rounded corners.</p>
     </div>
     <div class="bs-example bs-example-images">
-      <img data-src="holder.js/140x140" class="img-rounded">
-      <img data-src="holder.js/140x140" class="img-circle">
-      <img data-src="holder.js/140x140" class="img-thumbnail">
+      <img data-src="holder.js/140x140" class="img-rounded" alt="A generic square placeholder image with rounded corners">
+      <img data-src="holder.js/140x140" class="img-circle" alt="A generic square placeholder image where only the portion within the circle circumscribed about said square is visible">
+      <img data-src="holder.js/140x140" class="img-thumbnail" alt="A generic square placeholder image with a white border around it, making it resemble a photograph taken with an old instant camera">
     </div>
 {% highlight html %}
-<img src="..." class="img-rounded">
-<img src="..." class="img-circle">
-<img src="..." class="img-thumbnail">
+<img src="..." alt="..." class="img-rounded">
+<img src="..." alt="..." class="img-circle">
+<img src="..." alt="..." class="img-thumbnail">
 {% endhighlight %}
 
     <div class="bs-callout bs-callout-warning">