Go to UserCP on the main toolbar at the top of the page. You'll notice two things here of interest. One is the options for Profile Picture, and farther down is the one for your Avatar settings.
You want the Avatar one. Now, all you need to do is save a jpeg onto your computer, that conforms to the maximum avatar size (pixel dimensions, and file size). I forget what those settings are off hand, but it will show you on the avatar upload page.

That should be it.

Hello,
i have posted the new thread here on zbrush.but i have a problem in showing the thumbnail in my thread.so can you please guide me through this?the thumbnail is not for the images,i m getting images here no problem in that,but getting problem in showing the image in my name thumbnail.