Piece of cake.
Make sure the Avatar you want to use is somewhere on your PC where you can easily find it.
- Go into to "Update Profile".
- Scroll down to Avatar
- Make sure "yes" is checked under "enable avatar"
- Click the browse button
- Navigate to where your avatar image file is saved on your PC
- Click on the avatar image file
- Click on "open"
- Click the "Update" button
It takes a little while before it will appear. The avatar images are actually stored on the FSM servers (which is different than posting pics, that requires a link to where the image is on a photo hosting site). Also, make sure the pic size isn't too big. You may have to reduce the size of the image.