Go into My Gallery.. click on the picture you want to use. Then click on Edit Picture. Then click on save as.. Type the name and at the very bottom it says reduce picture. Click on the drop box and click on Thumbnail. That will be small enough to use. Good luck..

Once your ready to put it in here... you click on your profile, avatar, click yes to the part where it says use custom profile and then click browse, find the new picture and VOILA! Let me know if you need any more help.. Good luck.