I'm genuinely surprised at how many people are in some way put off by the images. I wonder if there's a cultural thing about people who dislike dogs generally. These kind of comment streams are always a good reminder about how you can never anticipate other people's responses.
I'd surely crack that joke with close friends, but at work I would absolutely not risk that, especially if I don't know whether the receiving party has a similar humor. Compare that with http.cat:
It's a cute picture of cats. It might get an eyeroll, yes, but nobody is going to accuse me of an inappropriate joke for that. 226 is even worse and I know quite a few female colleagues which would probably not take it well, irrespective of it being a picture of a dog.