Click on the little man above the search box. The first line of the next page will read: 'change your public profile'. The next page will show your profile. Scroll all the way down to the bottom of the page and you'll see your avatar with the acorn. To change it, simply browse your 'pictures' folder for the one you want, click the picture, then click 'open' and it will be added automatically, deleting the acorn.
