As someone who had a part in the avatar that Gary Gygax used, I have to say that it bothers me a little also.
There was a discussion about something to help people identify Gary, and he did not want to use a photo so I cut out the Futurama pic, and he liked it. Someone else edited it slightly afterwards if I'm remembering correctly, but unfortunately all those posts appear to have been lost.
Anyways peoples avatars are special, they can come to represent the person. To me that avatar "is" Gary Gygax on these boards. Perhaps because it's much easier to identify posts by the avatar than names. I remember being thrown off when Hobo/Joshua Dyal had stopped using his snarky looking guy for awhile. Or imagine if someone else started using PirateCats avatar, it could cause confusion and I'm sure people wouldn't appreciate it.
Like others have already said, I know no disrespect or harm was intended, but still I'm glad to hear that you'll change yours Gentlegamer. In my opinion it should be retired sort of like a hall-of-famer's jersey in sports.