make sure you are using the newest official BBM out there, and it may take a little bit of time to change..... it has to update your picture and everyone you are attached to via BBM
Posted from my CrackBerry at wapforums.crackberry.com
don't do anything it's not you it's bbm if you notice on your updates i bet you don't see anyones recent updates saying changed display picture.... it happened to me the other day i changed my pic 10 times thinking it was me... turns out it was bbm... just make sure u have the newest version and if you do than just wait a few hours or so and if you don't have the newest bbm backup your contacts through your options and upgrade your bbm