[css-nav-1] Modify the introduction section

* Change the image

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -117,19 +117,19 @@ This ability to move around the page directionally is called <dfn lt="spatial na
 <a>Spatial navigation</a> can be useful for a web page built using a grid-like layout,
 or other predominantly non linear layouts.
 The figure below represents a photo gallery arranged in a grid layout.
-If the user presses the <code class=key>Tab</code> key to move focus,
-they need to press the key many times to reach the desired element.
-Also, the grid layout may arrange the layout of elements independently of their source order.
-Therefore sequential navigation using the <code class=key>Tab</code> key makes focus navigation unpredictable.
-In contrast, <a>spatial navigation</a> moves the focus among focusable elements
-depending on their position
-allowing it to address problems encountered with sequential navigation.
+If the user presses the <code class=key>Tab</code> key to move focus around the images, 
+they need to press the key many times to reach the desired image element.
 
 <figure>
-    <img alt="When elements are laid out in a grid pattern, spatial navigation makes it much easier to predict and control where focus should move to." src="images/spatnav-enable.png" style="width: 500px;"/>
-    <figcaption>Application using a grid-like layout.</figcaption>
+    <img alt="When elements are laid out in a grid pattern, spatial navigation makes it much easier to predict and control where focus should move to." src="images/gallery-app.png" style="width: 500px;"/>
+    <figcaption>Photo gallery application example using a grid layout</figcaption>
 </figure>
 
+Also, <a>spatial navigation</a> moves the focus to the predictable element for users 
+because it moves the focus among focusable elements depending on their position. 
+Sometimes elements on the page aren’t arranged independently of their source order.
+Therefore unlike <a>spatial navigation</a>, sequential navigation using the <code class=key>Tab</code> key makes focus navigation unpredictable.
+
 While arrow keys are naturally suited to control spatial navigation,
 no previous specification describes how that should work,
 or how it may be controlled.