Maybe they just think its a random photo of any old dog? Assuming you didn't provide any documentation with it. Perhaps theres a book you could get out on it, I've been told before by someone a published book has more validity than online stuff (to be honest thats not always strictly true though). But if theres no book then
http://en.wikipedia.org/wiki/Balto is a good place to start or other sites.
If you do show them documented proof thought and they still don't believe you, I agree with Kari to ignore them. As they would then obviously just be winding you up...
I don't see why they think you'd lie about this though....