There doesn't seem to be any differentiation in the IMG tag src attributes to indicate which is the larger and which is the smaller images. They're all named the same, e.g.
If you change this to reflect a difference in the images, either via path or image name, you can add a click handler to the anchor tags wrapping the "little images" (or simply the images themselves) to change the "large image" src attribute to be the "large version" if the little image source.
<!-- Big Image -->
<img src="css/images/img1.jpg" alt="" />
<a href="#"><img src="css/images/img2.jpg" alt="" /></a>
I'd give the "little image" IMG tags an attribute to reference them by, e.g.
This will make it a bit easier for jQuery to target these images.
<img src="css/images/img_2.jpg" rel="littles" alt="" />