I'm pretty much a dummy when it comes to this sort of thing, but when the site went through its last big update I found some good suggestions and links on someone's thread. I finally found some sites which deal with avatars, and I found mine.
I just had another look (I bookmarked mine way back when) and found the main site.
Marg has a great suggestion. I had never seen that site before.
If you want a custom one, you need to do what I said before.
ok, you need to google the word avatar. you will get a bunch of hits for websites that have avatars.
There are tons of different sites, I just tossed one out there for an example.
You find one that you like. Now what you have to do is have a way for this message board to recognize that image. The image needs a url. Some sites already assign the images a url, but most do not. If they don', you need to first download the one you like. Now it will be in your "my pictures". Then you have to find an image hosting site (I use photobucket). If you have never used one before, it's pretty easy. You need to sign up. What this does is assign a url to any image you like. So you load your new avatar into your image hosting site, and you will get a url for it.
NOW, you can copy & paste that url into the "my stuff" page here on this message board, just like you did with the Y! avatar.
Shop around, there are thousands of avatars out there. Have fun!