1. Log in.
2. Click on your Profile tab on the right side of the screen.
3. Click on Edit Avatar in the profile sidebar on the left.
4. Scroll down a bit and click Browse to open a file browser on your computer.
5. Locate an image file to use and select it.
6. Click Upload Avatar. Browse away or refresh the page to see your new image anywhere your avatar is displayed.

Note: if the image you want to upload is bigger than 20KB, you'll need to first process/crop/resize it before uploading.