why is there..

I guess the reason is because it is a picture of a sleepy dog. I happen to find sleeping animals quite cute, but understand why it may not be what comes to mind for some people in terms of a category icon...

Probably because the dog looks tired. There are a lot of pictures that are kind of okay, but if you look closer they actually are related to the category it's under. I don't think it's weird, especially if you're a dog lover. I think the animal people will respond well to that. If you don't want it as your default then you can change it.

That's Garfield the cat. He's a big sleeper.