Not quite the right section for this (!), but -
1. Click UserCP at the top of the page
2. Click Edit Avatar on the Left hand side of the screen
3. You have two options from this point -
i. Upload from an internet location - simply fill in the location of the image in the box provided and hit the 'save changes' button
ii. Upload from you PC - press the 'browse' button and then use the explorer window that pops up to locate and select your image. Press 'Open' and then 'save changes' back on the main page
Whichever method you choose your picture must be within the set dimensions - 100x100 pixels or 19.5kb - whichever is smallest