Yeah, they seem to be static images stored on the server, rather than generated dynamically via, say, Google Search. Personally I would have gone via that route. Have the word "kitten" and the pixel size as parameters, and serve a random photo out of the first 50 results.
This would mean no caching, so your computer would have to download the image every time, even if it was the same as one you'd already seen.
Unless you're only using these images for client demos, it can be very slow and frustrating waiting for them to load every time you refresh while trying to do html, css or js tweaks.
