Well i believe they are being made on the fly. Not on the fly as when your request them but maybe once a day or so.
There images are the same as all video games stats that are created.
I started to mess around with this once but never finished it.
What i was looking into doing was reading an .xml file that had all my stats and then using vb.net you can take an existing image and then create an object of that image, place the text on the image and then save the newly created image (something like that).
The approach i was going to take was pretty much the same thing as water marking an image. I was able to do all this in a web page using asp.net. Asp (by its self) can not do this.
Now is that how they did it? Who knows, there may be many ways to do this.