Here's what ya have to do:
1) Right-click on the image you want to use and choose the option 'save picture as'. Save it to your desktop under the default settings so that it's easy to find.
2) Go to your 'user CP'
3) Click on 'edit avatar'
4) Go to the 'custom avatar' section and look for the part that says 'option 2 - upload image from your computer'. Click on the browse button next to it
5) Browse for the file. Then click on the 'save changes' button at the bottom of the page
That should be it done. Again let me know
P.S. Sorry for the late reply. I posted this ages ago but I posted it on my vistor messages by mistake. lol.