Go to your USERCP (on the blue bar under the banner, click on usercp).
On the menu on the left hand side click on edit avatar.
Scroll down to the bottom of that page and it will have a section for adding a custom avatar (now you have over 25 posts you can add a custom avy).
Either upload one (100 x 100 pixels with a size limit of 30kb max) from your PC or insert the web location of the image and click save changes.
You should then have a custom avatar
.
If not, pm me and Ill help you a bit more.