Ask HN:How Animoto.com works?

I am Keen on understanding how Animoto works. Users can upload images(or syncs with flickr/picasa etc) and a music file, Animoto generates a video slideshow for you. Heard that it creates 720P video using GPU cloud...thats amazing. My understanding is it takes this image and audio data and gives it to a video Encoder on GPU cloud and generates an encoded video. On my macbook, using iphoto its just a click away. But I am not able to grasp how it works on the web? Do you guys have any idea how complex is this to achieve and what web technologies are required for this??