I found the solution.
In the result, the thumbnail url format is: http://ts4.mm.bing.net/th?id=H.4798308230825487%CF%80d=15.1
The encoded character %CF%80 is the Pi.
If I remove the Pi from the end of the url, it works.
I'm not sure, this is the best way, but it works. By the way I still don't understand why url contains Pi...