You can also put the picture in our own galleries, then use the url for this picture in the avatar section. Click on Forum profile information, Click on I have my own pic:, Paste the http:// in the space. Click Change Profile at the bottom of the page. This will give you a good sized avatar.
If you upload it as an avatar from the profile section, instead of as a regular picture, it will automatically be resized to a smaller avatar. This is the easiest way to put it in, but it will be the smaller avatar most people don't like.