Anytime a site uses Javascript to perform an operation you can view the source code for that site to see how they are doing it. In this particular case, the code is embedded right in the page.

Basically what is happening here is that the thumbnail image is loaded and a link is displayed below the image. When the link is "Click to enlarge" the Javascript takes over to display the also-preloaded large image. The effects being used to make it "grow" are in the Javascript and it is called "scaling". When you "Click to shrink" the image, the reverse happens. The larger image is replaced with the thumbnail again and the scaling process gives it that visual "shrinking" effect. The text is changed upon each operation as well.